Its because most of altcoins (hundreds of them) are really exactly filled with uncertainties. Whenever I look through websites of new altcoins I can find dozens of red flags. Coin distribution maths are generally very bad and sometimes look intentional so developers can get rich over investors. Sometimes whitepaper are very messy and amateurish. Only top 10-20 maybe 30 altcoins feel safe to invest.
It more like every altcoin out there is a scam or a rugpull or a shitcoin that has been abandoned, making it equivalent to a scam.
Whitepapers copied from another project, advisors being fake/stock images, non existant products and vaporware - this is a day in the office for altcoins that have been recently launched. You dont want to be a part of this shitshow unless you are not having anything else to do.
If you ask me, I would feel safe max with bitcoin but to stretch it, I can accept the top five altcoins. They have been performing well to stay in those positions for years which is why I suggest users to give a notice period of at least two years if not more to a newly launched project before investing.