Because they want to look all grown up. When they deny players' requests they feel mature, like parents that won't give kids candies. Parents know better. After all, this is a beta test of a computer game. Serious stuff. No time to waste on something silly as skins. As opposed to throwing shit all over NCR, sending people to Glow, or locking harmless but awkward topics.
ME: ...but I am fully aware of skins' limitations, so it's my decision.
GM: You mean OUR decision.
Up yours then, you patronizing dick.