Why what is the case? Your question isn't clear.

The bolt holes usually strip because the bolts have been over-tightened at some time. Add to this an overheat, and the aluminum of the crankcase can be bonded to the threads of the bolt. With the weakening caused by over-tightening, and the heat, the aluminum can strip out the next time the bolt's removed.

Overheating can also help stripping because an overheat will overstretch the steel bolts. When the engine cools, the head's loose. If the head bolts are tightened when the engine's still warm, they'll be loose again when the engine cools. They wind up being repeatedly torqued a bit too much to prevent loosening, and the holes strip out as a result.