This has likely been covered before but I did not see it

What is it that goes on with Skyrim that it requires the cache to be verified and updated due to verification issues to remain playable?

I have had games on my pc for years that have never had files need replacing so this game cache verification seems very odd to me.

It's to ensure that no base files were changed and to ensure that no base files got corrupted and to make sure they're all there.

I get that but the question is "why do they become corrupted"? I have never experienced this with other games, modded or not.

Sometimes it'll happen when Windows crashes, sometimes the HDD accidentally overwrites part of a file, etc. It can happen for a number of reasons.