The problem with having all of the privateers in their own class, that would leave, in the case of 2005, the premiere class with only 7 people (2 Honda, 3 Suzuki, 2 Ducati). Pretty boring if you ask me.
Unfortunately, the AMA is very successful in spite of itself.
Gary Edmonson was the only one that had a chance to create a premiere-style series when he tried to split off from the AMA and create NASB. However, that quickly went to the courts and, even though he was found in favor and got a $3M judgement, his series never materialized and the AMA came out ahead.