Webbing would not guarantee a stable stop. In order for it to not be destroyed by the rocket exhaust the engine would have to cut out quite some distance above it, in the range of tens of metres or more, which means a long unpowered drop toward a hard surface and a net that will be much more likely to fail than a controlled landing. Rocket goes crack, knackered components, fuel everywhere, big boom.
To make the net robust enough to catch the rocket without tearing apart you'd have to make it of some highly elastic material that still has a very high tensile strength, which means you're then dropping your rocket several tens of metres onto a giant trampoline, which will guarantee that your rocket begins to ascend with the wrong parts pointing toward the sky and descend shortly afterwards in an unpredictable location. So, even if the net holds, you have something taller than a house and full of volatile fuel flinging itself about in random directions and making an uncontrolled landing. Assuming it doesn't explode, the engines will almost certainly be damaged beyond repair by the impact.
Your docking clamp idea suffers the same problems of height and heat and adds a shitton of unnecessary complexity as well, not to mention time and fuel use. The rocket would have to come to a stop to be grabbed by the clamp, which means it would have to hover in place, using more fuel than a controlled landing and obviating the clamp's entire purpose - if you can hover the rocket, you can land the rocket, and that means you don't need the clamp. The reason why it has to hover is simple: if the rocket is still moving, it will impart energy to the clamp, by which I mean it will damage a complex piece of machinery. That adds repair time. It adds risk. There's also the very likely outcome of the clamp damaging the rocket, which more than likely results in - again - a big boom, at which point you've not only lost your engines, but you've also destroyed your docking clamp and have to rebuild it.
This is all assuming you're on a stable surface too. On the barge, you'd either have that big net swinging around like a trebuchet or you'd have the clamp waving about like a gigantic bat just waiting to slap the returning stage out of the sky.
So that's why.
Biting the hand that feeds IT
