Because if he needs to do that, then presumably other people have the same problem, and I'd be reasonably sure at least some of them can't or aren't prepared to use wget, sed & grep to find things they did in another program.

Browsers already have history, so it seems to me it would be logical to extend that functionality to show how a page changed in response to an input somewhere in your history.

It's not easier. It's just easier than what we already have.