Because there's more precision and we are sitting together making the sharing of...

Because there's more precision and we are sitting together making the sharing of the touchpad more awkward then sharing the screen as an interface.

It's pretty simple.

I also have two kids under 12 and their first reaction to any screen is to use it as an interface and look confused when it doesn't work :D

(I use an MBP myself and do find the desire to have a touch interface like her Surface)