Money.

It is a win-win situation for both the shitcoin team and the exchange. How? The exchange will charge the owner of the shitcoin to give them a certain amount of Bitcoin in exchange of listing them on their exchange. More popular exchange = Higher listing price.

Now if the team's shitcoin got listed on an exchange, chances are they can do a pump and dump on it especially if it is on a popular exchange since many traders are using their exchange and there will be some traders who will trade their coin too. In this way, they can get more profit into it and when the time comes that they've had enough they will delist the coin and after that abandon it. Not all are doing it but most of them are doing it.