I suppose the same reason why we drive super luxo yachts vs horse carts
It's a good question, really.
Some of the reasons why I use a laptop with a USB GPS receiver:
-street maps with restaurants, hotels, POIs
-various Topo software for varying degree of drill down
-ability to interact with a keyboard is MUCH easier than a dinky touch screen and look for the A, B, C, keys
-Double duty as internet access during the trip or when you get to the destination. Check the email at Burger King while eating lunch.
-Able to download pics from camera cards on the fly.
-USB ports to charge phones, cameras, etc
-a 13" screen is much easier to see than a 3.5' screen. Especially topo mapping.
-Much easier to insert Waypoints using keyboard. For example:
Great camping spot, next to the brook on the west side, vs
C_spot_1
-Passenger can watch movies to past the time.
I'm sure there are more but these come to mind right now...