because for some reason, some people just don't really grasp the concept behind ebay (or auctions in general) - that, or they don't care about the price.
this isn't just for corals. i know a bunch of people who tell me "well, i just input in the max i'm willing to pay, and if i get it, great, if not, oh well"
except, you stand a much better chance of getting it cheaper, if you simply wait for the last minute to bid that maximum.