Several reasons.
1 - It's 4 blower, 60 HP and makes much less noise that my 20 HP, 2 blower Mark VII on-boards. My guess is that you don't get the echoing effect when it is outside.
2 - There is no dripping onto the dried car from the equipment, door, bldg. etc.
3 - Building a longer bldg. to house something inside, when outside is free, and a less harsh environment makes no sense.
4 - A friend in a nearby town put his inside after I advised him not to. I went through his once. It was a huge pain in the a$$ to creep through his, because it was close to the auto equipment, and my rear wheel was going through the treadle while I was trying to get through the dryer.
In the 10 years since I've had it, not once have I wished I had put it inside. Frankly, I wish I hadn't put the electrical panel inside. The box has rusted badly and is about to fall off the wall. I can't imagine a company like PECO would have put that cheapa$$ box with something that is to go into a car wash
Is there some benefit to having it inside the building? I don't see it.