It would be way more annoying for LHT countries to change to RHT than to switch from US units to metric, have to change literally every road sign, street light, road markings etc in the entire country.

Plus everyone would then be driving cars with the steering wheel on the wrong side for decades until the whole fleet got eventually replaced.

Countries have done it before, but I think there's a reason not many countries have done it since the mid 20th century when roads and cars were a lot simpler and there were fewer of them.

Switching to metric could be done gradually - replacing existing signs with metric ones only when they wear out, building new systems in metric while still using US units for the old ones.

There's no way to gradually switch from LHT to RHT, you have to somehow do a switchover in a day