There's a big difference between "writing your own engine" (a bottomless pit of features your game probably doesn't need) vs writing a 2D game with a framework

There's a big difference between "writing your own engine" (a bottomless pit of features your game probably doesn't need) vs writing a 2D game with a framework like SFML. Engines like Unity can be nice, but they can also be clunky and opaque and get in your way, especially if you're doing something outside the norm. It can be simpler to just write the code you need, as you need it. That said, of course you should use an off-the-shelf engine for a 3D game if you ever hope to finish it.