Some people order a lot, and some people may order less but the items may be heavy, like computers or musical instruments, or other heavy/bulky items which can't be found elsewhere (out of print, vintage, limited editions...). I think it's reasonable to expect your middleman to wait some time and group items, especially if you've been a good customer for several months. I believe it's supposed to be a more human and personalised service than your usual internet shopping website like Amazon, and that is usually reflected in the fees charged.
I still think the main issue is lack of space, and rules which they wouldn't mind overlooking, but they're rules so they stick to them. Besides, some services charge daily storage fees per item, so they're happy to keep them around