beacouse they aren't. A restrictive part is a cat with a bigger turbo, where a HUGE amount of air is filtered passing through a lot of cells;
If you make a whole exhaust with the same diameter all the way from turbo to the tip, and you would use mufflers with the inlet/outlet with the same diameter , there wouldn't any restriction for having mufflers. The smaller pipe diameter used from middle muffler to the tip is more restrictive, and it isn't noticable with a stock turbo. If you just cut the muffler and put a piece of pipe, won't be any 'improvement' than more sound. If you just make a whole bigger exhaust with stock turbo, won't be any 'improvement' than more sound.