Many altcoins are based on scams, they appear only to make money for themselves. Many projects have seized on investors' greed for profit and raised funds for their own interests, rather than for the development of the project and the interests of investors.
That is what the scheme is and the old users like me know how such scams run under the convenient cover of anonymity that bitcoin and the internet has to provide them with. The ones who fall for these scams are mostly the newbies who seeing the price of bitcoin cannot afford to buy whole number of bitcoins and end up buying thousands of shitcoins that are worth the same but will surely drop to zero in a few months.
Point is that there will be successful altcoins that reach near to what bitcoin has been but these are rare and they will rarely be picked up unless you are getting lucky. Every team seems to be working hard but below the table you never can be sure. Hence keeping bitcoin as the max investment yields the safer portfolio.
no doubt people who have been in crypto for a long time or have been here a long time are aware of such scams and they would just prefer to keep safer bitcoins and a few altcoins that have been around for a long time and have a good reputation.
for cases of beginners, they are often deceived because they are inspired by several altcoins that have soared like shitcoins, memecoins like Shiba Inu and Dogecoin. so they only see if all crypto will go up high later but without seeing how big the risk is. indeed choosing for the long term is good but a little bad if holding altcoins, especially if new altcoins.
so all the old folks who have been in crypto always recommend holding bitcoins. Even though the price is high, that's not an excuse, but we can buy it using the DCA method of collecting it until it becomes a lot.