> but when you create your own engine you are _fiercely_ aware of _everything_ it can and can't do
The problem is that if you start gamedev by making engines, then you aren't aware of what you need to do.
To give an example, if you make font rendering and looks blurry/pixelated, what now? Oh, and a simple 2D game takes 8 seconds to load, wonder why?
Meanwhile, if you have ever made a Unity game, chances are you already know the keywords "SDF" and "Texture compression", because you tore down an already big engine for optimizing your game and accidentally learned about what features a game needs.