i think the reason is actually because it costs nearly nothing to create these tokens on ETH whereas creating an actual cryptocurrency requires more effort and some money.
for example take the popular memcoin known as Doge. the dev had to copy bitcoin (or rather litecoin) and make changes to the protocol, test it and spend a lot of time making sure those changes didn't break anything. then release and have to mine it themselves to keep the coin alive for some time before others get onboard.
in comparison a token creator just writes a much shorter script on top of ethereum and doesn't spend any time testing or spending any money running any miner or anything like that. they jut release that tiny code and have a tokne!