I believe many others feel differently, but personally, I find the Go mascot a bit scary. Dunno the reason, but it's something that I can't easily get used when reading the articles on Go.
FWIW, Russ Cox's GitHub icon[1] is similarly scary to me.
I've never been a fan of "creepy pervert gerbil" either. I wasn't familiar with Russ's even creepier rabbit-thing, but now the gerbil makes more sense.
Well, since the gopher images are Creative Commons Attributions 3.0 licensed[1], you could make your own and sell them or give them away if you wanted. I think the point is to promote the language, not to monetize it, and making it available through the Google merchandise store was probably easiest for those involved.
FWIW, Russ Cox's GitHub icon[1] is similarly scary to me.
[1] https://github.com/rsc