When they keep a high expectation of making profit within a particular time and later get disappointed. Everyone with different expectations and motive as to why they are investing in bitcoin and also some started investment with the wrong narrative about bitcoin because it is mostly believed that if you invest and hold for some time, the price of bitcoin will increase. It is actually true to make profit in bitcoin, but no fixed time to that since the market is very unpredictable and fluctuates, thats why it is mostly preferred to hold for long term. The time investor will be expecting a bull, the market will be experiencing a dip which get so many disappointed, since they had high expectations believing profit is guaranteed as at that time meanwhile profit isnt guaranteed in bitcoin.
Secondly as youve also said, investors who use bitcoin investment to try luck will also end up not able to invest for long term and I believe this set of people who try their luck with bitcoin are not even sure of bitcoin and also dont really know much about it because bitcoin isnt a game or gambling that works with luck, it only the condition of the market that will determine the outcome.
When you decide to start any investment with high expecation of profit, you are automatically putting yourself in a pressure because achieving your goal will be your main target, and because of that, you can become greedy at some point just because you have high expectation of making money within some period of time.
Any investment comes with it's risks, same as Bitcoin investment too. Just because we say that Bitcoin is safe if you invested and hold for long term doesen't mean that there are no risks involved in it's investment, we recently see incidents that occured on some hardware wallets we see as the most safe wallets, it is part of the risk of investing in ditdal currency like Bitcoin same as other physical investments also have their own risk.
If you want to invest for future purpose, you don't have to panic. If you buy BTC in the right way and get at least Electrum wallet to save it, and keep your seed phrase safe, you don't to place it based on luck, your money will be saved and will maintain it's value for long.