>but it's much harder to come up with novel and fun gameplay ideas

I'm going to give an analogy to fiction writing. You ever notice how seemingly everyone has an idea for a story, but shockingly few of them ever execute on their ideas? Why is that?

Well, it's because the ideation stage is the easiest fucking part of writing a book! Sitting down to turn that idea into reality is a process that takes months/years. It requires a lot of concentrated effort, and a willingness to deal with the fact that your writing skills (probably) suck.

It's the same with video games. Probably more so, because the medium is much less restrictive than fiction writing.

Also, quick tangent on the topic of "novel" gameplay ideas--if you listen to successful creatives, some common advice they give is that focusing on being original is a noob trap. Ideas are cheap in the arts, and most things have already been done. It's the execution that matters, which ties back in to my point above.