Because we need to get enough surface area on the tires so they don't sink into soft sand. To get that surface area, they need big tires. They also need to be light so we can thrown them over our shoulders. These two factors drive what we have today.
The question you should probably be asking, is why don't we back our trailers in to the water to launch the boats?
I can understand the concept fully when we are talking pneumatic tyres and the need for ultra lightweight trolleys, obviously we have the problem where the 'floatation' is enhanced by the pneumatic tyres........
but what about the plastic barrel style ......
why are these sealed so that they too float ?
the only logical reason I can come up with is for assistance in structural strength of the lightweight plastic... otherwise I'm at a loss as to why.
So we 'need' a wheel that will move over sand, the pneumatic tyre and the plastic barrel both do this with the slight advantage going to the pneumatic tyre as it has some flex in the tyres sidewalls compared to the rigid plastic wheel.....
equals a slightly smoother ride for the boat

then the grey matter started grinding over.......
not wide enough, weight is unknown, and price wouldn't be all that friendly......

but great potential
