There are no tourists in my city. At least I usually don't see crowds of people on our streets. Rarely a few buses come at the biggest park and people just walk around. They don't throng like a herd. They are quiet and mostly senior.
People don't join a crowd themselves. Usually guides collect many people and lead them through a city. Larger the crowd means larger the profit. It's just a bussiness. Although they could tour through a city by different way. They could split sightseers into small groups and send them to diverse places. But there are disadvantages in case of small towns which have only one point of interest. And it would bring more problems for travel agencies because they would need to hire more guides and solve more logistic troubles.
What if a town's administration forbade travel agency from bringing more than 7 people at once? Probably, visiting that town would become more expensive for tourists or they would need to wait until other groups go away. In any case such a town would fall from favour with visitors and lose money.
We all know that tourists waste a lot of money: they buy souvenirs, food, tickets for transport and for some entertainments or shows (for examples, for bull's fights). They use public lavatories, buy umbrellas or sunglasses. Many people think they have to buy some clothes and then drag them back home. It doesn't matter that these clothes were made in China. Finally, they all need to live somewhere and they rent rooms in hotels. So, tourists are very powerfull source of profit for every town's economy and any administration should involve people in its town as much as possible.
Why do people behave like that? Why do they bring problems for locals and for other visitors? Several problems can be solved simply. For example, excursionists should use radio-guides instead of loudspeakers. But what about rudeness and arrogant manners? I think the reasons are an impunity and a herd instinct. Everybody's sure that he or she is situated for a short time in a town and nobody will follow and point the finger at him or her. This feeling is supported by other member's behaviour.
Tourist's breeding is a very complicated issue and I have no answer how to make it more agreeable.