They do it because they are looking "to get rich" and think one of these alts will have a huge run. It's also physiological in that ypu can buy some of these alts for pennies or less than pennies so they can buy a lot of coins. It's all relative but there truly is a theory that alts are cheaper. Also greed. Greed drives the entire meme market
Also the fact that there has been history of some low cap alts suddenly turns into multi billion market cap coin is what motivated them the most.
so many people actually looking for that kind of hidden gem and since such coin are quite frequent in the bullish market, people invests more and the market of alts become much more active with good gains and the cycle will be endless.
but I think the idea that bitcoin market cap is already too high that the volatility is low might also be huge contributor to the fact that there are many people who seeks volatility to profit off and alt serve exactly just that.
it all comes down to get rich quick scheme at the end of the day.