standard algorithms for single-source/single-dest pathfinding scale log-linearly in the size of the graph, so compared to other combinatorial optimisation problems, optimal pathfinding is incredibly easy for computers to do & scales pretty well to industrial-sized problems. computers can also do optimal pathfinding for problems that humans would not be able to solve easily (because the graphs don't easily embed in 2d or 3d, say, so we can't bring our vision systems to bear)

other combinatorial optimisation problems - like the traveling salesman you mention - are much harder than pathfinding to solve optimally or even approximately

>while a computer would take weeks or until the heat death of the universe to do it better.

I don't buy this, approximation algorithms are an entire field of CS, if you're OK with an approximate solution I'm sure computers could do that quickly as well.