I am no expert, but to me a timer on a webpage would mean that instead of a relatively simple page of text and pictures, there would have to be an actual program running on the page to count down the clock. This has two consequences to my mind.
Firstly, including a timer on EVERY item page would probably require at least 2-3 times as much server storage space from eBay, greatly increasing their overhead.
Secondly, the program code would be running (I think) on your computer and not eBay's servers, so due to timing differences and the communication lag wouldn't actually show the correct official time (although it might be close). And having the timer constantly check with eBay's clock would require much greater bandwidth - again more overhead.
Basically I reckon it is not worth eBay's time and effort, especially as it wouldn't actually really be used >95% of the time. Most people rarely sit there waiting for the auction to nearly end so they can jump in with a higher bid (although I admit I used to when my eBay habit was at its height!).