Why does it cost more than painting a room?

The paint is more expensive.
It takes more coats of paint.
It’s an awkward three-dimensional surface.

Does it take longer than painting a room?

Yes, because it requires more coats, and it needs to dry longer in between the coats of paint.

Do I have to remove the doors?

Cabinets can be painted with the doors on or off. It just depends on how nice you want it to look when it’s done.
If you remove the doors, it takes more time, it’s going to take up more space, and your kitchen is going to be apart for longer. The positive side is that all of the hardware stays clean, so it has a nicer more professional look.
If you leave the doors on, it’s a little bit faster, it doesn’t take up as much space in your house, but it’s not as clean, and paint will get on the hardware.

Do I have to empty the cabinets?

Yes, if you want the inside painted which some people do.
No, if everything is pushed back from the front edge of the cabinet and you only want the outside painted.

Before and After