Because we are humans.
And humans are made or designed to do things by learning. We learn by trying, watching, practice things,....
While doing this we make mistakes and learn from it.
Because our brain and physical ability are just like that. We need to forget things to move one with other things.
We also do things by importance.
Can everything be perfect?
No and Yes!
The weird thing is that most people want everything perfect but they have to fact that it can't be.
We need to be satisfied with what we have and what we can with all the mistakes we make and limitations we have.