They are unable to do so.
Most aircraft don't even have microwave ovens.
Not least, because of potential effects on avionics and sensors, not to mention power surge issues.
What they use for hot meal service is elaborate ovens made to heat many meals at once, provided they are packaged a particular way from the food supplier. The oven doesn't lend itself to general use for single meals.
There is also hot water spigot service for tea, which is how they heat a baby bottle. This might help with the ramen, but will get you nowhere with microwave fare such as burgers, pizza etc.