You have a valid point, one I have been convinced on for more than ten years. When planes first started getting really big, a few people tried tabs but for some reason they never caught on. I think it's mostly a paradigm. The big names all use multiple servos, so everybody else wants to. Big models could be finished cheaper (maybe, though the labor cost for the airframe would be greater) and maybe a little lighter with better aerodynamic counterbalancing and one servo per surface. But, many people validly worry about the loss of redundancy with only one servo. Also, (and this is a biggie and probably the main reason tabs didn't catch on) with the big 3D models doing blenders and walls and such, boost tabs will not help hold the surface in position against the massive drag forces present during those maneuvers. You would need nearly equal forward counterbalancing such as 25%-33% offset hinging on ailerons (which almost no-one does) and large leading counterbalances on elevator to make up for the reduced holding power of only one servo. Also, with the aileron supported at fewer points, it would have to be built stiffer and a little heavier to prevent flutter, though servo tabs can help with that particular issue if istalled well away from the servo arm. It would really be accomplishing the same thing with a different technique. Offset ailerons hinges are more complex and heavier but you can lose servos if you use them. So if you're willing to do the work, you can save money on servos, wires, match-boxes, servo programmers, and batteries. It's just plain faster and cheaper for the ARF builders to make butt aileron joints than it is to go like full-scale and make offset aileron hinges.