Theres a couple of ways to look at why some wouldn't want their builds seen half done.
Some creators, like artists, don't like their work seen before its finished, for various reasons and its subjective each creator. They may want to hide their techniques, or like Reggie mentioned simply don't like showing their process.
I'm 50/50 on the fence about it, in some ways I absolutely hate half finished work being seen, but don't really mind sharing it with friends and teaching.
The other side is the dark side of the force, Copyboting. Which of course can happen any time, but if my work is stolen, i want it stolen finished with my signatures attached if that makes sense.
otherwise I agree, once the main sim is kind of set up,, no reason to not be friendly.
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