I think making a game engine is less like making your own knives and more like having a herb garden at a restaurant.
There are some games that are just not possible using someone else's engine. Consider Factorio, non-Euclidean geometry, the early MMOs, or anything else which pushes the boundaries of what's possible. On the other hand...are you really making a game like that? Similarly, there are good reasons why you'd want to grow your own herbs, and the deep knowledge of growing conditions really does add to your ability to understand why your parsley is bitter this year, but for most restaurants it wouldn't pay off.
There are some games that are just not possible using someone else's engine. Consider Factorio, non-Euclidean geometry, the early MMOs, or anything else which pushes the boundaries of what's possible. On the other hand...are you really making a game like that? Similarly, there are good reasons why you'd want to grow your own herbs, and the deep knowledge of growing conditions really does add to your ability to understand why your parsley is bitter this year, but for most restaurants it wouldn't pay off.