simple reason, because they have a market interest. and their project is used in the long term. indeed there is no guarantee that the altcoin project will last a very long time. so I won't hold altcoins, or trade them for short term gains.
when a new project produces its altcoin, there will be an increase in the number of altcoins in the future. and also some of them will disappear. when they no longer update the project and lose the market.