People like designing and making games, so they design and make games… Making the engine (or underlying framework, whatever) is just not necessary for that goal.
There are relatively few game designs (especially with indie scope!) where a custom engine is truly necessary.
For the tinker/hacker types, there are plenty of things to do at a “lower level” within the engine itself (note: I don’t necessarily mean modifying the source code). There are all kinds of different ways to structure things within the confines of the engine, and lots of other ways of building frameworks and GUI tools within the engine too.
I think it's also easy to conflate game development as one big thing. 100 people could say that they want to make games and all mean 100 very different things. I think for some people, like you mentioned, they ostensibly want to "make games" but also just like to tinker of engine-flavored tech, and that's totally okay.
There are relatively few game designs (especially with indie scope!) where a custom engine is truly necessary.
For the tinker/hacker types, there are plenty of things to do at a “lower level” within the engine itself (note: I don’t necessarily mean modifying the source code). There are all kinds of different ways to structure things within the confines of the engine, and lots of other ways of building frameworks and GUI tools within the engine too.