As I already explained, because it’s easier to reach in cramped spaces like a plane or other small tables or, gasp, your lap. When the laptop is close to you, using the touch pad is awkward where you have to do that t-Rex arm thing to reach it, but the screen is by definition approximately a forearm’s length away due to the geometry of a laptop. So the screen is right where your hands are naturally when your elbows are at your side. It’s also more direct. If you want to scroll two inches, you move it two inches with your finger - with a track pad it’s not 1:1. You can also directly scroll any window, with a track pad you have to click an inactive window first before the scroll gesture gets routed the right way. Touch is fast and intuitive.