Percival P. Cassidy wrote:

I have lived both in Australia and in Taiwan. In both
countries, bathroom floors had drains to avoid serious
flooding in the case of overflows. ..

Why have I never encountered such in US bathrooms?

When I was in OZ last year, I was surprised to see these
drains in the floor. But none of the showers had doors,
just pull around curtains. Did not see a bathtub in any
house or motel.

My bathtubs and my sinks have overflow valves and there's
a plunger next to every toilet.

Also in the States, most county/city plumbing codes does
not allow waste water to be dumped on to the ground.

Dick