Well for one because the Vertex Interpolator node exists so there isn’t much need to go through the process of doing that manually. But to each their own workflow.
And as your comment implies not all math can be done per vertex. All that said I agree that math is almost always cheaper than memory and per vertex over per pixel whenever possible.
I’m just making the point that just because sampling the second time isn’t as expensive as the first, it’s not open season. Once again, you very frequently see performance concerns about having to sample the same texture multiple times when using techniques that require it like tri-planar, anti-tiling, blurs, PoM, etc.
I am really glad that modern GPUs are as good as they are. The hoops you used to have to jump through to get something to look good and run decent even as recently as PS4. We’re really spoiled these days.