That may seem true at first glance, but with door types, what happens once other mechanics get involved? Sure, you can recreate an Exact Door with a pretty simple contraption… until you want to make puzzles that involve using Master Keys to bypass Exact Doors, or cursing Exact Doors, or making copies of Exact Doors… you get the idea. A simple one-time check to make sure you have exactly a specific Key count isn’t hard, but when you actually want it to behave like a door - with everything that entails - suddenly it’s not so easy. (Refresh Keys don’t have this problem since they’re not doors, though, so they’re easily recreate-able)
You can always add more mechanisms to allow these things. The "door" will end up becoming large and impractical after adding a few of these, so I see your point.
Here's an example of what I mean, using the I Wanna Lockpick Editor:
The 14(!) added doors incorporate positive real Master Keys into the design of the Exact Door.
(Also, it's still possible to make simple special Pure Doors, since they are immune to most effects.)
I've previously attempted this myself:
This is a (frozen) Exact 3 Orange Door, which should work correctly with positive Master Keys, Brown Keys, and whether or not Orange is Starred (though it probably doesn't work if any of the other colors involved are Starred), and that Lockless door at the front is for frozen/eroded/painted purposes. If it wasn't obvious, you start at the left end of the image.
Here's a simpler version that doesn't work with Master Keys but still supports Brown, Star, and Auras:
I imagine trying to implement door copies would require a whole restructuring into a sigificantly more complex design... and all this complexity is just for Exact Doors, which are one of my simpler ideas! Imagine how hard it would get to implement my actually complex ideas (Partial Blast Doors, Starry/Forceful, Laggy Glitch, etc.) via these kinds of mechanisms...