Timestamps have the property of being easily comparable; you can always tell if one file is older then the other. If you were to use hashes for the same purpose, you'd have to keep a database of expected hashes, and comparing them would be a less trivial task, etc. It's doable, but it would be a very differently designed (and much more computationally expensive) program then make.

I'd say the sad part is that nix really works well when the toolchain does caching transparently. But to deliver good DX outside of nix, you kind of want great porcelain tooling that handles everything behind the scenes - downloading of libraries, building said libraries, linking everything together. Sometimes people choose to just embed a whole programming language to make their build system work e.g. gradle. Cargo just does everything. Nix then can't really granularly build everything piece by piece when building rust crates with Cargo - you just get to rebuild every dependency any time the derivation is built and any one input changed. I wonder how much less time would've been wasted if newer languages chose to build on top of nix. Of course, nix would need to become slightly more compatible with Windows and other OSes for this to be practical.