It's called "work-hardening", when you bend a piece of metal back and forth. The bend part gets harder and more brittle until it breaks. This is a function of metallurgy, not the fault of a bike or material. Steel, titanium, they all do it. It's why planes will disintegrate mid-air, like the F-15s, that passenger plane over Hawaii, etc - the planes stretch and shrink when they pressurize and depressurize at altitude. Do this many thousands of times and the metal will fail.

I forget the company name - Problem Solvers? Wheel Manufacturing? - but some company makes a bazillion dropouts and they even made, at least they used to make, dropouts for frames that didn't have replaceable dropouts. You saw off the broken dropout at a certain point and bolt the replacement on. After that you have a replaceable dropout frame.