Many people do work on their own engines, and do not have a game to show for it - which isn't to say they aren't cool projects, but it does involve reinventing many things people have already done, and also takes away time that could be spent prototyping

Extremely valid question. Most devs should not attempt making their own engine. But 2 caveats:

- learning is also fun

- Most engines (by force or by available tutorials and docs) railroad the end user into some specific genres (in my experience: side scroller, FPS). If you wander too far away and understand the risk/complexity, it might be worth considering.