I think you see so many rebuys on the Merge sites because it is easier for them to offer guaranteed money in those tournaments with lower traffic than what you see on the bigger sites.

Stars could (and I believe still does) offer $4k and $5k guarantees on $1.10 buy in tournies because they knew that they would get over 5,000 entrants.

Even with the recent growth, there are still no more than 10,000 playing at any one time across the entire Merge network. In order for them to offer guaranteed money in a tournament they almost have to make the tournament a rebuy in order to reach the guarantee.

Hopefully with continued growth you'll see fewer rebuys as time goes on.