Why is there a strict rule on how to hold a baseball bat, or pool stick, or anything else in sports?

I guess I didn't know till recently that there is a specific way to hold a baseball bat and it doesn't even make sense. Technically you are supposed to have both hands at the very bottom right next to each other. I would not feel comfortable holding it like that. I hold mine with one hand at the bottom, and the other one up a few inches. Now I know why there are so many foul balls in baseball because they aren't even holding the bat right. My 5-year-old nephew started Little League baseball and it made me so mad that everyone tries to tell him to hold it that specific way. But he never hits the ball hardly like that. I have taken notice when he holds the bat the way he wants to he does really good. Geeze, why can't parents and coaches have the brain to tell players to hold the equipment however it feels comfortable for THEM. Holding the bat like that is not going to work for every single person. Same thing with holding my pool stick. People have tried to tell me to position this position that and it just doesn't work out for me, I hold my pool stick weird but I can play a lot better like that