Many people doing this for safe and secure investment. But if you want to multiple your capital then you must investment in altcoins. Cause bitcoin always has large marketcap and it need a huge investor to skyrocket from this current price, when altcoins can easily skyrocket within night. So you can't force anyone to buy bitcoin only, its personal choice. Well i suggest you to keep your bitcoin in your wallet cause if anyhow exchange get scam or problem happened with your account (kyc, violence issue) then It'll be difficult to safe your investment.
at the end of the day it comes down with the fact that some might take risk for the sake of increasing profit they wanna generate off their investment its just as simple as that.
though honestly, such thing could be simply prevented by diversification, but then again some people are more eager in multiplying their money instantly to go such lengths of investing their whole investment to the
altcoins which sometimes is gonna be turning out to be really wrong considering the fact that altcoin is just not as safe as bitcoin in general.
but then again, some people might be limited in terms of capital so it only make sense that some people would prefer to invest in altcoin than bitcoin, as far as I know bitcoin only good for people with at least more than $10k investment and even that is already considered small and insignificant enough with bitcoin investments.