There wouldn't be much point paying millions for another site to simply play on the same network, especially since most Full Tilt players probably already had a Full Tilt account.
Poker Stars and Full Tilt are two powerful brands in online poker industry. Before that "tragic event", Full Tilt was the 2nd player on the market.
Some players are brand loyal. As they did in the past, some have chosen to play there because of the name, the software, the games... because everything that FT means. They could have played on PS but they didn't.
But for creating a huge poker room, where you could find 1000 tables at any stake at any time, I see no reason for this merger.