They are created by trending and they die by trending too. When a new trend appears, many new altcoin projects will be created to hype the trend. When that trend is cooling down, loses its hype momentum, the trend is gone, their developers will leave their projects too. Trending projects are more sensitive to become dead projects.
Not all, though; some of those projects manage to survive for a long time, and some can stand the test of time, but from the majority, only a minimum number of those tokens can survive. There are some old existing altcoins too, like ETH, BNB, and their likes, which only follow the market trend; they dip during the bear market and also surge high during the bull market. It is better to invest in such coins that are more assured than just some new hyped meme coins. If you must invest in those hyped tokens, you need to be smart and properly educated about those coins.