Seriously what are so many devs thinking that they got the next big multiplayer hit?

Because it's their game. They won't screw up. They won't be the ones to have a failing game. They made it, so they can now influence it in the way to make it successful.

Also, I think it's just the massive potential. Look at any of these multiplayer games that have gotten popular. The failure chance is big, but also the potential gains are incredible.

It´s just lazy greed. Making a mediocre MP game is way easier than coming up with a good story, a worthy AI and a proper campaign for an average game. And they´re hoping to make a bundle off the whales in micro-transactions