Test

Several differences.

The kettle is a lot closer to the cup and has thermal mass. I generally pour with the water at the bottom still boiling slightly. It takes time to get the water out of a microwave and pour it into the pot/cup.

A kettle gets the water to a boil, then stops. A microwave is likely to either overheat (and so waste energy) or under heat, and so be too cold.

The kettle water can be used to easily warm the pot, if needed, before it fully boils.

Kettles are a lot more consistent for water heating than microwaves. Add water, click switch, wait for boil and switch clicking off. I would give good odds I could walk into most people's kitchens and work their kettle without instruction or significant effort.

Kettles can't superheat water.