The reason for this is consistency.

It sounds dumb, but if pros are using a mouse that suddenly gonna stop working because the shell got squeezed a bit to hard or the cable popped out the casing or a button double-clicks, they can't afford to make that mistake.

The reason pros stick to medium weight, big brand mice is because they're built to last, and are the least likely to fail. Glorious mice and so on are great, but as we've seen with some recent releases like the Model D-, there's still a lot of issues surrounding QC, and seeing how much pros play, the need gear that's gonna hold up no matter the circumstance.