Full time 4WD was used in many trucks and SUV's in the 1980's but it fell out of favor as the gas mileage was a lot worse and they were less reliable. The more that is turning full time the more power that is needed and the more wear and tear.
Even where AWD was used as with the Audi, Jeep, and Toyota sedans it has been a very limited success and discontinued after a few years in most cases due to lack of customer interest.
The only time I have wanted full time 4WD on the highway is when I am going from dry pavement to snow covered pavement continually as it is easier than switching into and out of 4WD constantly. I sometime encounter that on Hwy 80 in the mountains around Lake Tahoe but that is about it. Anywhere else it is snow or a dirt road and 4WD stays engaged until I am on dry pavement.
I used to have to get out of my truck and engage the manual hubs and get back into the truck and start down the road again and then do the same when back on dry pavement. Then auto engaging hubs came on the scene and that was a blessing, but I still had to work a transfer case lever and watch my speed and manually shift into and out of 4WD. Now my truck and SUV both have a dial I turn to engage or disengage 4WD and I do not find that any more inconvenient than having to manually turn on the windshield wipers when it starts to rain.