Simple answer is fingertips vs palm

Golf grips are wider at the top, don't really know what else to say

OK,
Now this is an interesting question. I'm thinking of all of the stick & ball sports and golf is the only one where the ball is stationary allowing more of a (sort of) controlled swing. Your'e not running around and maneuvering where the club may not be your immediate focus.

No knob sports; all racquet sports (tennis, squash, racquetball), field hockey, ice hockey (except goalies), lacrosse, cricket, polo and croquet.

Knob sports; Baseball and some hockey sticks.

So really its only baseball bats that universally have large round knobs.

Turns out the baseball knob does nothing to help the batter hold onto the bat and the knob actually causes injuries to the player's hand (broken hamate bones)! who knew?

blog.axebat.com/.../

So if your buddy really wants to get a knob for his golf club here's a company that can help him out!

https://buttendz.com/shop/

It could be because you carry golf clubs around in a bag, and a knob on the end of each club would make it more difficult to insert and withdraw clubs from the bag. Withdrawing clubs would be particularly difficult as the knobs would get stuck under the other knobs in hanging out at the bottom of the bag.