Microwave with IR thermal sensor is available at least here in Japan since around 2000. Weight or humidity sensor is also used as a cheaper (and older) option.

Microwave with IR thermal sensor is available at least here in Japan since around 2000. Weight or humidity sensor is also used as a cheaper (and older) option. Those sensors are used for auto heating rather than specifying time. I don't know why it's not widely used by everyone even in developed world.