Why don't all sets have the same pieces available? Why are some missing?

Why don't all sets have the same pieces available? Why are some missing their mirrored counter-part? For example, Classic Brick Wall Top Half Round Arch Left doesn't have a Right version, and you can't just flip it to have it align properly? Why is that a thing? Are there any Thememaker Toolkit items that adds missing parts from the existing sets?
 
I've always wondered the same and it's something that I felt always made the game seem only half-finished, that and the changeable scaling for some pieces.
 
It's a design choice and one I have personally never agreed with. it's very limiting but they choose to only offer certain pieces they felt were critical for that theme in order to save resources and offer more variety I suppose. They went for breadth at the cost of depth.
 
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