Because auctions go on for days or weeks, and the more people bid at the start, the higher the overall price tends to go.
Very true. The more bids an auction gets early on, the more noticed and "heated" that auction becomes, and when an auction becomes hot then its final price is ALWAYS higher. Therefore, it stands to reason for the buyer not to show his interest in the auction until the very last moments, in order to get the other buyers in cold temper (and without time to react, i.e., "become hot about the auction") and therefore win the auction at a lower price.
It may not be 100% rational, but it's the way the (passionate) human mind works.
Consider this: I was winning this auction at a dirty cheap price (0.99 UK Pounds), and for six days that auction had been life with only one bid: my bid. It was not a particularly "hot item", in fact it is quite vintage, so I thought there was not real interest from other buyers in it. My highest bid was set at 5 UK Pounds. So I got relaxed.
Then in the last seconds, someone came snipping and got the item for 5.5 UK Pounds, and I had no time to react. This way, the snipper got the item cheap, because he avoided turning the auction into a "hot auction".
Could I have bidded early on with a higher maximum bid? YES. But I was also bidding on other ZIP disk auctions, and I didn't want to win them all and end up with a thrillion ZIP disks, I was just shopping for the cheapest priced auction of ZIP disks.
To tell the truth, I was expecting the snipper in the last seconds, and that's why I did take the screen snapshots: I wanted to see if my fear of a last second snipper was founded, or not. And it certainly was!