Worst of all, they wouldn't be much fun.
As a loader, either you're pulling rounds from a rack and then shove them into a hole where they disappear with a bang. Or you'd swap coax barrels and ammo belts. All of that are distinctly physical activities where a mouse/3D interface are utterly pointless. Or, if you're not doing the physical thing, you could observe out of the hatch like the commander, without being able to influence much (unlike the commander's position), except that you have the AA MG and can get whacked by some unseen sniper while you look all badass.

The driver's position might be slightly more fun, but it's largely in there, better in some vehicles than in others, but again the main point of the driver's position in SB Pro is to support the training of the commander, not to train the driver himself. For that, you have dedicated driving simulators with a substantially higher complexity. Last but not least, the driver's position is also more physical than the Gunner's or commander's, which is why motion platforms add so much immersion to any driving simulation. But the typical player doesn't have a motion platform at home, and in any case SB Pro isn't a racing game. Spintires is probably the best driving simulation for military/offroad vehicles there is, and I admit that it has its own appeal, but then the maps are just like 5x5km² where we need to be able to simulate 150x150km², so the technology in Spintires could not possible be adapted 1:1 without running into severe scaling issues in multiple dimensions.