IIRC, top-level domains were originally intended to be more rigid, the way .gov and .edu are reliably for goverment sites and schools. So just having .uk wouldn’t mean anything. It only signifies that a .co, .org, or .gov domain was registered in the UK. By the time other countries’ registries became noteworthy, the whole com-net-org distinction was almost entirely lost, so they didn’t bother forcing those subdivisions of their top-level domain.
The reason American DNS doesn’t apply a .us extension everywhere is because we invented it. Neener neener.
IIRC, top-level domains were originally intended to be more rigid, the way .gov and .edu are reliably for goverment sites and schools. So just having .uk wouldn’t mean anything. It only signifies that a .co, .org, or .gov domain was registered in the UK. By the time other countries’ registries became noteworthy, the whole com-net-org distinction was almost entirely lost, so they didn’t bother forcing those subdivisions of their top-level domain.
The reason American DNS doesn’t apply a .us extension everywhere is because we invented it. Neener neener.