So why the hell do they keep coming back on the markets?
It's because there is a market for them. They won't be back if there's no market for them but due to those investors that are eyeing them and thinking that they can profit from them, they come back. There's no solution for that but the investors will eventually have to turn away from them when it's no longer profitable and for the side of these project makers, they'll just revert the attention of their investors by doing some updates or making a new project that will look interested to the same investors that they've got.
It is really that simple, for people like us it doesn't really make a lot of sense that coins that are completely useless and that we know have no future keep disappearing and then reappearing with a new name but with the same characteristics, but at the end it is a matter of demand.
This is similar to what happens with illegal drugs, we know about all the side effects that they produce on your body and how bad they are for your health, so in a perfect world there would be no demand for those substances, but instead we see the opposite, people want them and since that is the case someone is willing to supply them, and shitcoins follow the same logic, they are completely unnecessary and they are bad for your economic health, but since people want them then someone is willing to supply them.