Exchanges list coins because they make a profit (and a good one) out of listing fees and because this listing will attract the community of that particular coin which "speculates" a potential for that particular... altcoin...
However, my not financial advice would be the following:
It is no coincidence that all institutions and hedge funds are actually paying attention to Bitcoin and consider the other altcoins as speculative assets that do not worth a thing...
The logic is the following (please correct me if I am wrong or you think there is something that I am missing)
The main thing altcoins are trying to compete with, is speed of delivering a transaction, believing that Bitcoin is slow so they are better... But Bitcoin is not even competing for the fastest transaction, it is competing with stores of value assets (such as gold).... This means, that altcoins are actually competing against VISA, Paypal, Maestro etc... and do you know why VISA Maestro etc laughed on the thread of cryptos? Because no user (including you and me) will use crypto for the day-to-day transactions and instead will prefer the current system which actually works just fine. It is FAST, reliable, protected from malware etc.
The problem with the current system, therefore,
is not the speed, but the limitless supply of money. Bitcoin fixes this by having a fixed supply.
Why altcoins are considered shitcoins?
1: For a token to succeed in having value, there needs to be LIQUIDITY. (EXAMPLE: why there is not much trading on land? Because the market is IL-IQUID, meaning that though it has value, it is hard to sell land as there are not many buyers and it takes time....)
2: For a token to succeed, it needs to solve an actual problem... What problem are altcoins solving when:
SOV -> Bitcoin, Gold, Silver etc
TXSPEED -> VISA, MAESTRO, PayPal
So name it ICO, DeFI, IEO or whatever, unless they actually solve a REAL problem, I doubt we will see any of them succeeding.
PS: Blockchain is the next big thing, but it can be used without issuing a token.
why will they list that coin at the first place if that coin is fake or dont exist ? they inspect it first and they provide rules that those only the coins can enter on thier exchange . shitcoins do have a value to so i dont consider them a coin that dont exist .
for your second question , is that a volume your refering to ? even the top exchanges are also suspecte d before to have a fake volume on some of the coins that they list . i think exchangers them selves cant monitor that easily . they dont lie because its not thier fault but except maybe to the fake exchange because fake volume isnt the only thing that they can do .
