Why are 120 V general receptacle circuits limited to 20 A?
Why 20A? Receptacle design constraints, most likely

The reason why general receptacle circuits top out at 20A is because the notion of a duplex receptacle doesn't work for larger plug sizes (just can't fit enough meat in there at a suitable spacing for it to work), nor does the idea of a "shared" receptacle (T-slot) that can accept 15A or 20A plugs, as well as the inability to get "feed through" functionality for a larger receptacle size.  (The parts required just become too large and cumbersome.)

There is a tricky way to get what you're really after, though

If you want to get more than 20A to a large number of receptacles in a space like a kitchen, though, there is a way to do this under the Code.  What you do, though, instead of running a fat cable directly to receptacles, is run that same fat feeder cable to a suitably-sized track busway, which then has bus plugs plugged into it that have fuses or breakers and standard 15/20A receptacles in them.  (Think of it like lighting track, only beefier -- it's common in industrial and lab applications where reconfiguration is a need.)

For a kitchen, I could see this working with the busway mounted under the upper cabinets and against the wall, with strip lighting attached to the front of the busway if you wished to have that as well.  Each bus plug would likely contain a 20A fuse and a Dual Function (AF/GF) receptacle (or a 20A Dual Function circuit breaker, if you don't mind the plugin units needing to be larger), and you'd want to run the feeder using metal-clad cable or wires in conduit instead of NM to mitigate against arc faults in the busway feeder.