The old ring circuit wiring in the UK did not adequately protect the wires in the walls against thermal overload. If you drew too much current from a single outlet, you could overheat the wires without blowing a fuse or tripping a breaker. So fuses were added to the plugs, since rewiring was impractical and reducing the house’s fuse or breaker rating would be annoying.

In the US and elsewhere, equivalent protection comes from correctly sized breakers in the panel.

> In the US and elsewhere, equivalent protection comes from correctly sized breakers in the panel.

That’s a common misconception. Your circuit breakers are rated to protect the wires in your wall—nothing else. The wires you plug into your wall receptacles are rarely rated to handle full capacity of that circuit breaker and can be overloaded without tripping that breaker.

A common example of this is an extension cord that breaks off into multiple receptacles on the end. Depending on the quality of the cable, you might be able to draw 20A through that cable and start a fire without ever tripping your breaker.

If each plug is fitted with a fuse that matches the rest of the cable, this issue is largely mitigated. That’s why Christmas lights often have fuses in the plugs: those cables aren’t able to handle the maximum load that your circuit breaker and in-wall wiring can tolerate.

The NEC does have restrictions on the size of a breaker supplying different types of receptacles. 20 amps through, say, a 18AWG extension cord shouldn’t be that dangerous as long as the cord uses high temperature insulation. But your point is good: a fuse protecting the cord would be a very sensible feature. (Even better would be ground and/or arc fault protection in the cord, but that gets expensive.)