Why must we constantly look back with perfect hindsight and see what we should have done?
Because we are human and have emotions.
I often say, "Man, I wish I could do that over." But normally, that is followed by, "Oh, well, it's too late now. No need to fret over it."
I think that is what prompted your question. You don't like, or don't understand, why some people worry about things in the past. Or why people attempt to fix everything that they think is wrong.
People are human. <shrug>
must we constantly look back with perfect hindsight and see what we should have done?
The erroneous assumption, the illusion, is that hindsight is perfect. If 'we had done things differently' there would have been a different series of events arriving at a different outcome.