Sure you can build an engine if you want- the reason so much stuff is unity is because it handles the cross platform build/deploy for you and is already a well established engine/platform.

If you’re trying to make a game- use an engine. If you’re trying to make an engine, don’t get distracted with making a game. They’re very different things these days.

If you’re just doing this to learn and want to make a game from scratch, then do that- but the cross platform is a pain.