> you can’t successfully monetize

this wasn't the goal of modding.

but there will be a point you'll wish you did.

So much has to go right for a new game to see even moderate success. In addition to the programming, you need an art director to give your game a coherent style, 2D texture artists, 3D model and terrain artists, UI designers, music composers, narrative writers, etc, and on top of that you need a compelling universe and concepts for all of these people to work from. And then once that's all done, you need competent marketing so people actually know about this game so they can want to play it.

By comparison, with a pre-existing game much of this is already out of the way and amateurs can get pretty far by just kitbashing existing assets and occasionally mimicking them when creating new assets. Marketing can be as simple as, "this thing you liked, but more, in the way you want it". It's a much smaller lift.