My guess is the cost vs performance gain for most just isn’t worth it. I’d imagine you’re looking at $3k ish by the time you figure front and rear gears, install kits, and labor to set them up, even if you R&R your own diffs and mail them off.
Obviously if you’re adding an ARB or rebuilding a worn out diff that would be the time to change gears, but as an “upgrade” the juice just isn’t worth the squeeze.
Honestly I think I’d rather have an extra 30 hp. My LR3 on 32’s doesn’t have the guts to pull my enclosed trailer in 6th at 70 mph, so I sit in 5th and burn way more fuel. I think 30 hp would be enough to get it done based on messing with various speed, gear, rpm settings.