What could cause a 5 year old valves to corrode so quickly
This doesn’t look like corrosion, primarily, but like buildup of minerals from small leaks. The valve’s thread may not have been sealed properly (or not been tightened correctly), hard to say under the buildup. The weld seems to be leaking a tiny bit – you could clean it with steel wool and put some talcum on it for easier observation.

JFTR, I agree that this is not normal after five years. I don’t know where in the world you live and if you could still demand a repair after five years, but this looks like sloppy work. That can happen in one or two places, but not all over the house.