Why are there so many rules about table manners?

Well one big reason is that eating among other people is a social activity. In any social activity, there are guidelines for behavior. Mostly these guidelines are intended to make the social activity pleasant or fun for all the people involved. 
For table manners, the rules are noticeably different among different cultures. Still, there are some common themes across cultures, for what is considered polite manners while eating. One common theme is to ensure that every person at the meal gets to eat. You can see this in the American table manner rule that nobody should not start eating until every person has been served their meal. In Mexican culture, any child who is nearby an outdoor party should be offered some of the party food. Another common theme about manners is that you should enjoy the food and the company, and not appear to simply be gobbling down as much food as possible. There are a lot of negative words to describe behavior that looks like eating too much or too fast, to be considered polite: being piggish, wolfing your food, gobbling, scarfing. 
One rule that helps enforce slow eating, in American table manners, is that you shouldn't butter a whole piece of bread before eating it. Instead, you should tear the bread into smaller pieces, and butter only the piece you will eat next. Breaking the bread in half is considered a minimum level of politeness. Also, it's considered polite to leave one bite of each dish on your plate. If you eat everything, it could signal that you were just hungry, and eating everything available! But if you leave one bite, it signals "I liked it, but had enough". Americans also slow down eating by using both their fork and their knife in their right hands. This means that whenever you need to cut your food, you have to put down your fork, pick it up in your left hand, and pick up your knife in your right hand to cut your food. Before taking the bite, you have to do the opposite: put down your knife, pick up your fork in your right hand, and take your bite. This slows down eating a lot. (This rule is in contrast to table manners for the English, who keep their fork in their left hand and their knife in their right hand). Also, when eating soup, it's polite to push the spoon away from you, after filling it. Wipe the bottom of the spoon on the far side of your bowl, before bringing the spoonful of soup to your mouth. If you eat soup this way, you avoid scooping spoonfuls directly into your mouth -- which again, could look like you are just wolfing down food as fast as possible, instead of enjoying each mouthful.
There are also some table manner rules that provide signals to the servers or the waitstaff, without requiring verbal communications. For example, if you put down your fork pointing at 4:00 (with the handle pointing to the lower right of your plate), then it communicates "I'm still eating". But if you put your fork down, pointing at 10:00 or 11:00, then it communicates "I'm finished with this food". Then your server can take away your plate without interrupting the group conversation, which again is an example of making the social activity more pleasant for the participants. 
Overall, the rules of table manners evolved to make the social experience of eating a meal together a pleasant experience for everyone.