Because sometimes people get caught up in an auction and will increase their bid beyond what they initially intended to, and they will drive up the price of the item. A person in that situation can either drive up the cost of the item for you, or cost you a winning bid.

The problem is that if you make your high bid early, you may outbid somebody who wants the item and who has put in a bid that is lower than their top figure. Or, that person may have second thoughts and increase their top figure now that they've been outbid. So, when you bid early two things happen. You may invite people to keep bidding against you, in which case you may ultimately win but pay more because you've given people time to have second thoughts and increase thier max bid. The other thing that can happen is that person's second thoughts causes them to place a max bid that's greater than yours, in which case you don't get an item that you would have otherwise gotten had you placed your bid at the last second and prevented them from rethinking their max bid.

This happens A LOT of eBay. Some people get very caught up in the emotional aspect of the auction and will constantly readjust their max bid if they are outbid. It even makes some sense becuase the fact that there is a higher bid somewhat justifies the idea that the item is worth more. So, if you bid $3000 thinking that's fair, and then the auction goes up to $3,500 a bidder might say to themselves, "wow, this is worth more than I thought, so I'm willing to pay more now".

Bidding at the last second prevents that type of re-thinking (at least as far as your bidding is concerned).