Because it's illegal. For every cat they take off the car and replace with a hollow pipe they face a possible $5000 fine... and a TT has 4 cats... that's a $20,000 fine if they're busted. Why would they take a chance of being fined $20,000 to make $200-$300 from you? But there are some shops that will do it. However the pipes are crimp bent... meaning when they bend the pipes they do it by applying pressure to it which narrows the pipe in the bends. A 2.5 inch pipe will go down to 2.0 - 2.25 inches where it's bent. That's not good for exhaust flow. The name brand testpipes are mandrel bent which means they're the same size all the way through...including the bends. Mandrel bending machines are super expensive ($50,000+) so muffler shops dont have them. You get what you pay for... you can get a set of used mandrel bent testpipes for $250 or so.
