Anonymity isn't one of key principles of Bitcoin, if you not use Bitcoin mixer or Coinjoin, you're still can get traced because Bitcoin use a public blockchain where anyone can see your address, transaction, how much you hold, where your coins come from etc.

Many people are still supporting centralized exchanges because they find it's more convenient to use and they feel safe to trust a top centralized exchanges. They're not entirely understand and use Bitcoin like the purpose, but they only focus about making money.

You know it much more convenient to have transactions to examples you've mentioned above OP. But still people could track your addresses which could cause to many criminal activities suich as fraud and scams. People still prefer to have centralized system as some legitimate exchanges could guarantee the safety of your funds and can ensure you to transact with legit people. Imagine putting your funds in not centralized manner and it's up to your decisions if your fudns would be safe.

Traders uses CEX for easy exchange and to trade,because cex makes it more convenient,easy and cheaper for them. So many people uses CEX because they are lazy and can't take the responsibility of keeping their bitcoin safe in a noncustodial wallet. Lack of knowledge on how bitcoin is designed and how to store it is also another reason. Bitcoin is decentralized in nature and should be used in that way. You can buy any kind of cryptocurrency in Cex because they have all type of cryptocurrency.

Because they view the bitcoin as an investment, regardless of the way the Bitcoin is designed, but the abuse of use (using something other than what was made for it) is famous in almost everything and therefore the profits are what these people move.

Also, we cannot launch a specific template for the reason for using bitcoin, even if Satoshi returns and said: Do not use bitcoin except for a specific purpose, people will not listen to him/them.

Yes, bitcoin was initially created as a payment method that could work without a 3rd party. But nowadays, people are considering and using bitcoin as an investment, they are less concerned with the function and utility of bitcoin but only interested in profit and convenience for them. And CEX provides all the services that investors need in the most convenient way. 
Moreover, the market is not only bitcoin, but we also have altcoins, and if you want to invest, buy, sell, and trade altcoins, you must go with CEX. It can be said that it plays an important role in the market whether you use it or not because people still need it.

Can you name a more convenient way of buying and selling Bitcoin aside from buying it from Centralized exchanges?

I doubt you can't say because there's no other way that is consider convenient is more convenient than buying it on Centralized exchanges. I mean yes Bitcoin has been created so that people can transact without any middleman on it, but nowadays, there are many people who are scamming these innocent holders of Bitcoin, thus they find a way to buy more Bitcoin in a safer way, and the way that they see is to buy thru these centralized exchanges.

Why do investors retain their Bitcoin there? It isn't advisable to retain, and store any coins in exchanges especially in the centralized ones, but for some investors, it's a way for them to sell their coins way faster if they needed it. I mean if they store it in a hardware wallet, then suddenly the price of Bitcoin spiked up and they choose to sell it, they need to wait a few minutes because of the transaction, and in that span, price of Bitcoin might go low already, so I guess that's the reason why they are storing it there.

Another reason is that they're traders. It's pretty obvious that traders will really store their Bitcoins on Centralized exchanges. Why do people use CEX? Because it's convenient, easy to buy, easy to sell, and trusted (only a few).

Traders and investors require faster and more comprehensive services for their activities. If DEXs are not able to meet those needs, then CEXs will remain the best option. However, not all feature-rich CEXs can serve as a home for crypto traders and investors, but only those CEXs that have official permits and are supervised by relevant security agencies, both at the international and national levels. Additionally, pay attention to the reputation of the cryptocurrency exchange because every exchange has the potential to suddenly confiscate user funds.

Always be mindful of how you store your money because when you keep your funds in a custodial wallet, your money is managed by someone else who could potentially compromise its security. So, be cautious even if you're using the most well-known exchange, as their security is not foolproof.

I think convenient is a subjective matter, I don't think it's not convenient to buy Bitcoin from decentralized exchange.

You're need to create your account by submitting your email and password, then confirm the code which send into your email.

In centralized exchange, you need to verify your KYC and they will ask you to wait for maximum 7 days, they can reject it and ask you to submit more clear version, let's say you've successfully verify your account, in the next month or year, they can demand more pictures, if you can't do it, you can't use the exchange.

In decentralized exchange, you don't need to verify anything and you can link your bank account or other payment, then you can start to trade!

In general, people's need is convenient, fast, multi-functional, and cheap fees, and centralized exchanges provide all those features. In addition, people have been used to centralized services from banks for hundreds of years, so it is difficult to change their habits, and it is not uncommon for people to choose CEX over DEX. In addition, bitcoin today is not only used as a currency, it is used for investment purposes, buying and selling is also a way for people to make profits, and CEX is where you can trade. CEX offers many facilities while DEX has too many limitations.

I wish it was that easy. Have you tried buying bitcoin via Bisq, which is generally considered to be the best decentralized exchange?

First, you have to already have some bitcoin in order buy/sell, which means its not convenient for first time buyers. 2nd, volume is so low that you will wait for a couple of days for your order to go through and if you want to deal with bigger amounts you will lose a lot.