Because I've got 100 psi at an incredible flow for as long as the tank lasts (or freezes I guess, but I haven't experienced that yet). One $15 fill-up lasts an entire weekend of impact use (take off travel tires at campsite, put on wheeling tires, take off rear tires at wheeling destination, put on boggers, then reverse everything). Plus all the airing up and maybe re-seating a tire or 2. :D
I got a used steel tank (at a local welding supply) and a 100 psi unadjustable regulator (online from a balloon company) for about $100 total invested. Add a $10 compressor for emergencies and I have all the "air" I'll need. :D