Arguably the reason is making a game engine is easier than making a game. Obviously creating all of Unity/Unreal is a ton of work. But, to make a game engoine you have a checklist of features (gameloop, rendering, audio, input, ...) each of which you can break into smaller features. It gives you a free todo list, each item on the list has generally known solution, and makes it feel like you're making lots of progress as you check things off and see systems come up. That progress feels great. And you are making progress but you're making progress making a game engine, not a game.

A game on the other hand is a blank sheet. There is no checklist. It's way harder to make progress on a game.