[Tomte]

If you hard-code everything in pixels, you have the same implicit dependency as with rem. It's a design, after all! Things have to "fit together". You're not changing every element's font size to an individual value and on a whim.

But now you have to keep all those relevant "main font sizes" in your head and do constant divisions. rem units give you the ratio directly.

[meerita]

Correct me, but I can tell what 16px is, not 1rem. 1rem can be 16px, 18px, 2000px. When you see tons of relative units, you need to doble check what it's going on. With pixels you see explicit values, so no double checkings.

[codedokode]

Yes, but a typical change request would be something like "make this menu font size larger", and in this case having menu independent from other parts of the page is better. Also, if you build a framework with variables, dependence on root font size, it would be complicated and most devs won't like to spend time learning it and thinking how to solve their task within it. Simple is better here.