1. Because it enhances the fun, the experience, releases inhibitions that lead to more people stepping out of their norm, which definitely leads to more cheering/yelling.
2. Maybe there are 10 total people that can't in that stadium, every one of the rest of us can. This is what is called a straw man. It has no impact on question 1.
3. Of course people can. People can also enjoy salt/pepper on their steak, or french fries, or whatever....it doesn't mean it is necessary for enjoyment. It enhances the enjoyment.
Same reason alcohol is introduced at parties, to lower social inhibitions and increase cooperative activities like dancing, clapping, cheering, yelling....hell just saying "hi, how are you."
It appears you have an inherent disdain for alcohol, and maybe you believe you have a good reason for doing so. Fine, live your life, let others live theirs.
Edit to add question: Are you questions concerned about the "illegality" of bringing the alcohol in? Would you ask the same questions at a concert or NFL game?