Because for each image that you upload that is more stress on our servers to put it out everytime you post. This way it saves on our bandwidth a little bit. Having the sort of stuff we do for downloads we get hit with a lot of bandwidth. The more we can save the better off we are. Last month Guru said we had transfered 1.5TB of data from our servers. That is a lot of data and it costs a pretty penny to keep the servers up like that. So that is why we ask for them to be hosted elsewhere.
It's not a big deal for a file or two here and there, especially with only several thousand users. But every byte adds up in the long-run, and 1 image should be enough for "free & reliable" hassle-free hosting here, and it's not unreasonable to have users needing more than one image use another service.