Because they haven't had to ... yet.
Pantech and Acer just signed amended tablet deals a few weeks ago, the first ever mention in a license.
In July 2011 Apple was weighing whether to purchase InterDigital during the SA. Just like they are now weighing whether to purchase patent pools. Apple was listed by dealReporter as a likely suspect and it was hinted in the call today that the usual suspects are included again for the pool being sold.
If what Bill says, what is motivating others to reach a license agreement is the potential for the patents to be picked up by a rival only Apple or Samsung likely have that ability to strike fear in the hearts of others.
Apple may be the buyer of the patents combined with a license agreement for tablets as Bill has alluded too as a potential arrangement. Since we are late in round one of bidding as per dealReporter ... we will get better clarity once the sale is done and could start the dominoes to fall.