For me, it’s a couple of things. First, gestures are awesome and not easily reproducible with a mouse. My keyboard is full of shortcuts for other things, I don’t want to take up hotkeys for something my trackpad can do.
Second, it’s consistent across all my computing devices. My laptop experience is identical to the desktop experience and I don’t have to carry a mouse in my backpack for my laptop.
Finally, my hands are closer, and on the same plane, as my keyboard. With a mouse, my hand has to move from a small claw shape back to flat for typing. With a trackpad, my hand stays flatter.
Bonus, I need less space for computing. I know you can adjust the sensitivity for mice, and even for trackpads, but in practice, for me personally, I tend to need way more space when using a mouse. I still play computer games so it’s not like I don’t own one, but for productivity, I’ll use a trackpad when possible.
Good for you, I hope your egocentrism continues to serve you well.
You really can't think of a reason why someone might prefer using an alternative input method?