Because something is cheap is not always an indication to buy, cheap things are jot always good/profitable. It is always important you do your research, find out the reason why a certain asset has gone down in price before you make the decision to buy or not. If you buy blindly or impulsively, you might loose your money.
If something is cheap and there is a reason for it to go down, then you might be worried. There have been some coins in crypto world that went cheaper and cheaper and eventually basically died, or there are stuff in fiat world where things went cheaper and cheaper and eventually got destroyed as well.
However one thing is for sure, bitcoin is not like that and I believe ethereum and bnb is not like that neither but you can argue against that, however bitcoin is definitely like that. Bitcoin went cheaper "just for now" 100% guaranteed, maybe it will go up in the next month, maybe it will go up 3 years later, but one thing is for sure that it will go up again. Only reason not to buy it right now is the fact that it may drop even more before it goes up and that is the part people are not sure about, everyone knows that bitcoin will be 50k+ one more time, but they do not know if it will be 20k before it is 50k+ again.