Why does US Code not allow a 15A single receptacle on a 20A circuit?
Let me give some framework around what ThreePhaseEel said.
I can understand wondering that, since UK appliances all have fuses in their plugs, and I bet the fuses are sized to protect the appliance.
The US doesn't have those. It depends on reasonably close matching between receptacle and fuse/breaker protection. Thus, the general rule is that receptacles must match breakers: 15A must be 15A, 30A must be 30A, and 50A must be 50A.
Now there are 2 exceptions.  To brush aside the other one, NEMA stubbornly refuses* to define a standard for a 40A receptacle, so 40A circuits must out of sheer necessity use 50A receps.
The other is allowing 15A receps on 20A circuits.  (the reverse is out of the question).   Note that it's already the case that 15A appliances with 15A plugs are allowed on 20A circuits; just take a look at the socket keying.

NEMA 6 is our standard for 240V appliances; the same rules apply.  See how 20A sockets are keyed to accept 15A plugs.  Even on a 1-socket dedicated 20A circuit, 15A appliances are allowed.
Really, we have two separate use-cases here.
General-use branch circuits.  As you know, the ubiquitous American recep is the NEMA 5-15.  Most of the time, those are made as duplex (two socket) receps, and are on circuits that have many sockets - typically 3 to 12 yokes (a yoke is the physical chassis that fits in a standard 1-gang box) so 6-24 sockets.  And most things plugged into them are low current - fans, cell phone chargers, TVs, etc.  In this &quot;general use appliance&quot; application, long experience is that 20A is &quot;close enough&quot; to 15A that such appliances are unlikely to find useful protection by being fused at 15A rather than 20A.  UL wouldn't approve an appliance that senstitive, anyway, or would require it to have internal fusing.  (note that US Christmas light strings have fuses in the plug - yes, our dainty little plugs).

Dedicated circuits.  Some appliances (think: built-in microwaves; air conditioners) are such heavy draw that they cannot share a circuit - i.e. they are running near circuit limits, or if continuous, near 80% of circuit limit  Yet it is reasonable not to hardwire them and have them cord-and-plug connected. (e.g. a window air conditioner that might be removed every winter, back when they were inefficient).  UL requires such circuits state in their instructions that they require a dedicated circuit.  That is where simplex (1-socket) receptacles are typically used.  When an appliance has such heavy draw, it makes more sense to require a close match between receptacle and breaker.  So effectively UL is dictating the circuit protection.
It's a bit of a tag-team operation: our Electrical Code requires you used approved equipment (110.2), approvers defer to UL, UL writes the appliance design rules, UL certifies the appliances as safe, UL certifies the instructions and labeling, and Code requires that you obey them (110.3b).

*The reason they won't is:  (rewind: in Europe, they standardized on 416V &quot;wye&quot;  or 240V leg-neutral - while it makes for a more dangerous residential power, it allows easy 3-phase distribution and is strong enough for industrial needs; this means you only need one power system)  Whereas Thomas Edison married the US to this lower-voltage split-phase thing.  To work around its limitations, we went to a wide variety of schemes: 208/120V &quot;wye&quot; 3-phase, 240V &quot;wild-leg&quot; delta, and 480V delta (277V leg-neutral), just to name a few. As a result we have a huge variety of socket families, and they must ALL reject each other. So finding a new pin arrangement is like trying to find a .COM domain name lol.