Put an empty bowl in and it comes out cool to the touch.
A bowl of soup and the bowl is crazy hot, because the soup warmed the bowl.
Heating a person with a microwave would make us pop as the water inside heats up faster than everything else. And blood is pretty fucking similar to water…
You could try it with an incredibly low dose, but you’d have to do it in carefully measured bursts. Even for hypothermia, warm baths are dangerous because the increase in body temp is too sudden and can fuck up the heart.
So there’s a whole bunch of risks and you’re still limited to what won’t freak out your heart. Warming just one part warms up the blood that’s there and send it through the cold parts. Like how you’re not supposed to immediately add water to a radiator after a car overheats. The thermic shock can cause massive problems.