Because it's just much more pleasant to use. Plus, being able to manipulate the mouse without taking my hands off the keyboard is also a big advantage.

I've never used a trackpad that did not have at least one of: bad palm rejection, inaccuracy around the edges, clicking that feels bad and/or makes the mouse cursor drift while trying to click.

Granted I have not used an expensive, modern laptop in a while and it could be much better now, but even then I just think the trackpoint is better UX and better posture, there's something uncomfortable about having to awkwardly squish your arm between you and the laptop to manipulate the cursor.