Do you find it helpful to try and understand why something like this is the case? I don't know the answer and I don't think there is an answer. The thing with languages is that they aren't necessarily logical. They weren't designed with a blank sheet of paper to allow the most efficient communication possible between human beings, they developed organically over time and the nature of that means they had to build on what already existed. And consequently some things have become embedded in the language that just don't seem to make sense sometimes if you think about them logically. These things just are. English, with its 'mongrel' origins and influences from several other languages perhaps has rather a lot of these moments, as you seem to find quite often.

Your question is valid. For the comment above ""she slipped out by the rear exit" (used as noun). Enter is always a verb. The only noun for "Enter" is the key on computer keyboard.

the point isn't to make something redundant, it's just that I learned in school that 'I exist a building through the exit' but for some reason, I 'enter the building through the entrance', which somewhat confuses me.

Why do you want to make a word redundant? English has around 900,000 words why does it want to switch some off?

Sorry, auto correction can be quite annoying