Why Do My Stubhub Tickets Have No Seat Numbers?

Accoring to StubHub - it is because they usually don't even have the tickets and because making this information availble would result in fewer tickets from sellers:

First, it's a privacy issue for many sellers. Requiring the disclosure of seat numbers would make many sellers unwilling or unable to sell their tickets at StubHub - and that would reduce the number of tickets available for you to purchase.

Second - and more important - we want to provide a level of guarantee for the tickets you purchase. Because most tickets are shipped directly from the seller and are never in our possession, guaranteeing exact seats is beyond our capability. For example, if your purchase 2 tickets from a listing of 6 tickets, we cannot guarantee which 2 of the 6 the seller will ship to you.