I believe most "OE" tires are somehow cheaper. Agree with the OP that tread life doesn't seem as good as aftermarket, even the same tire model. But it's only a ad hoc observation as I don't put much stock in tire life. Too many variables.
The nittos and kooks I can't comment on as I've never run either of them, but for new tires, consider shopping amazon. I just got a new set of Firestone MTs 275/65/20 delivered for $760. Not the tread you're looking for but there's some deals there. With mounting and lifetime siping I was just over $900 all in. Compared to somewhere around $1500 for comparable Toyos or Bfg from the tire shop.
Have bought every set of tires off Craigslist used for years til these. That's another option if you find some or are near enough a big city with lots of CL traffic.
Around here you could pick up a nearly new set of the Fierstones, takeoffs, for $500 or less. Takes the sting out of tire wear at that price.
2016 Ram 2500, MotorOps.ca EFIlive tuned, 5” turbo back, 6" lift on 37s
2017 Heartland Torque T29 - Sold.
Couple of Arctic Fox TCs - Sold