it would also create an immense point of leverage on the axle/bearing race. imagine this slip-on collar rigid guard as the horizontal blue bar in this:
the red arrow is the impact applied to the guard when you smack your bike onto a rail for a grind, whereas the triangle pivot is the contact area that this collar sits on the axle, against the inner bearing race. the resultant force applied to the bike (since its also where the collar rests against the inner race of the bearing) is the green arrow, which is effectively on the same spot as the triangle pivot in this case. the impact from the grind is, therefore, largely amplified thru leverage. this means you're creating a mechanism that's prone to deformation from a rock hit rock approach and long leverage, and the consequence of when that happens is a possible combination of fucked bearing/bearing race/hub wobble/axle damage.
Keeping everything in context, dont forget there is a 4.5in long lever attached to the very end of the axle + a fat internet nerd jumping up and down, smashing the end of it repeatedly into a concrete block. In the grand scheme of things the load from a small hub guard cantilevered off an inboard area of the axle, is going to be a fart in a jacuzzi when compared to the loads from the peg.
They're mostly good because they protect your spokes and they're a lot lower profile than the axle mounted ones.