If the kitchen has a window, you're relieved from this requirement somewhat. It is 100cfm intermittent , or 25 cfm continuous. If you have some other sort of mechanical or natural ventilation, you are permitted to have a ductless range hood. You still need a range hood, but if you've got something like a window, it just doesn't have to be hard ducted to the outdoors. M1502 of the IRC.
One of the nice things about the code is that we don't have to know why. They make it easy on us and just print the requirements in black and white.