what if making those engines is to just benefit me.
Like I said before, making actual games will benefit you more than creating an engine. Through creating an actual game, you learn how to make pleasing game mechanics and a fun, playable experience, as well as an appealing visual experience. This is most of what matters in the end.
hand them in as part of a resume for a programming job(doesnt have to be game dev). i think it would work much more than putting down a game made in unity or something.
If you're not going to be a game dev, you shouldn't be making game engines, period. If you are, employers will be much more impressed by many games made in Unity, moreso than some engine you spent a lot of time working on that you never actually used to make a game.
And believe it or not, a lot of commercial games actually use Unity and other popular engines, and hire people who know how to use said engines. They won't hire you because you know how to use an engine you coded yourself, and even if they're somehow impressed at your non-tangible work, you probably wouldn't be using that engine in the future at all, and you'd need to learn how to use something different entirely.