Yeah I think its probably because the 35's will fit underneath, unlike a Toyota 4Runner which cant even fit a 33. I have a 315/70R17 on a 17x9 wheel under my 2014 right now and it fit no problem at 37 psi. I don't think anyone who is serious about offroading an F150 is running anything larger than a 35 because then you are talking about a 4"+ lift. I was able to squeeze my 315/70's in with the stock suspension, even on +12mm offset wheels, but just barely and I do get a brief rub at the back of the drivers wheel well once in a while
I had to put spacers on the rear sway bar on my Lexus GX460(same frame as a 4Runner), and slide the trailer hitch back as far as possible just to stuff a deflated 285/70R17 under mine. Oh, and I have a ratchet strap compressing the tire against the hitch to get me just a little extra room. It still rubs on the panhard bar in certain situations. To my knowledge, i am the only one attempting to run a 285/70 in the spare location. So basically the Toyotas have to have a 32.5" or larger tire mounted to either the rear door or the bumper and therefore they have the options available via the aftermarket.