Two big reasons for manufacturers not reinforcing wheel wells is weight and cost. Enough steel to really be effective could weigh upwards of 50 lbs per wheel. That much fabricated steel could cost upwards of $150. Sure, some of you had something fabricated for less than $150, but is something from an HVAC shop going to be thick enough to prevent damage?
Is the manufacturer going to sell more RVs by putting $150 and 200 lbs into the interior, or by putting it into wheel well liners?
I have a bus conversion, and it does have wheel well liners made out of thick aluminum. I'm not convinced a tire failure couldn't still tear through the aluminum. I would rather have the same thickness in steel. I do have a Pressure Pro system.