The theoretical answer is that most bids arrive in the last few hours or even minutes of the listing. This means that you could be the winning bidder until the last second of the auction, then lose to a “snipe” bidder, by as little as a penny.
It is usually advised that bidders act late, make only one bid, make that the highest price they would consider, and remember to check shipping costs.*
The actual answer is that eBay takes a percentage of the winning bid. The more they can persuade you to bid, the more money they make.
This goes for the shipping cost too. Insurance and Confirmation of Delivery are only important to sellers. There is no reason for a buyer to pay extra for these services, but eBay and Paypal make more fees when bidders pay for them.
*This is even more important with the introduction of the wildly expensive Global Shipping Program.