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.
yes, you are right, because many altcoin that have sprung up at this time just want to steal investors money (although not all of them), but we must still be careful if we want to invest in altcoin, because if we choose the wrong altcoin then of course our money will be lost, but if we choose the right altcoin, then of course our money will increase very much (get a lot of profit).