Because when you put the crew in the vehicle the vehicle gets bigger and it becomes harder for the the vehicle to follow the soldiers (both own forces and enemy) on foot or 50 cc motocross bike.
The whole impetus behind UGVs is to take a load off dismounted soldiers so that they can advance farther, and faster, in complex terrain with weapons that give a higher probability of overmatch. One of the reasons tracks are popular is that they can go places wheels can't go, and, in some instances, places feet can't go.
Agreed x3.
You could drag a 120 on a pulk. But it would be easier if it had a few horses dragging it. Same with the 81. Its possible, as Goose Green demonstrated, but .....
Just like Wireless Ridge demonstrated the value to a small division (2 light brigades) of a couple of troops of toy tanks with popguns
Which brings me to the value of the MPFS vehicle.
I understand the value of a battalion of MPFS vehicles to a light division. It can be a very useful augmentation to light troops in the right circumstances. A penn'orth of heavy firepower at the local commander's fingertips can be very useful indeed when trying to maintain momentum (or deny it to the enemy).
My problem with the MPFS is that it is too heavy. It weighs half as much as an Abrams but the same as a LAV 6.0. That means that the advantage of the light force is diminished because it requires a bigger aircraft (and better runways) or a boat to get the MPFS into the field with the troops. A more flexible solution would have been something like the Scorpion with a 90mm low pressure cannon which weighs about as much as a JLTV or a Bv206 at 8 tonnes - all could be sling loaded forwards by a Chinook. All could traverse muddy swamps and piles of rubble.
The MPFS is still conceived as a tank by some rather than a gun. That is explicit in the design of the turret and its commonality with the Abrams turret. And yet we are concerned that the MPFS will be used like a tank, when driven by tankers, even though it is not a tank.
Why was the 105mm gun adopted rather than the 76 or 90s that were considered? For the same reason NATO ended up with the 7.62mm round. There were lots of them in storage. Peculiarly the Ukrainian kerfuffle is depleting the stores and consideration is being given to resurrecting 105mm production lines. Why not replace the 105 with the 90 LPF and lighten up the MPFS so that it could accompany the light troops more often?
And there is ample opportunity to convert light vehicles, even the size of the 4 tonne Wiesel, into Optionally Manned Vehicles that can be securely, remotely controlled with a tether (fiber optic by choice).
If you don't put crew in the vehicle you don't need armour and can carry more rounds on the gun.
I keep coming back to this
Drive the pickup as far as you can
Drive the ATV as far as you can
Walk as far as you can.