Because you suck and we hate you?
I think HK produces the "legacy" (my term ... don't quote me) items in batches. The USP 45 is, in my opinion, THE 45ACP to have. I imagine they produce limited quantities of spare mags and people like me who are admittedly fanboish about the gun buy all of them when they're reasonably priced.
RTG is good to go. Lots of years of ordering from him.