Why does scaling for UI elements in Unity need to be explicitly configured?

I recently started building my UI and ran into all sorts of scaling issues when viewing it in full screen mode (proportions/spacing/etc. completely changing, buttons appearing outside of their panels, etc.) via the "Maximize on Play" option.

I was able to get it to behave how I wanted it to behave by tweaking the Content Size Fitter, messing with the anchors, and using various combinations of layout groups. My question isn't really how to get UI scaling to work - rather, what I'm wondering is why all of this tweaking is necessary in the first place.

The entire rest of my game... the tiles, the sprites, the weapons... all of these things scale perfectly and don't get distorted in the slightest when I play in full screen mode. I didn't have to do anything special to make it this way - it just works.

Why is the UI not the same way? There must be some obvious technical reason for the Unity devs to have built this intricate UI scaling system, but I'm just not grasping why they can't just scale it automatically like they do for everything else.