Sublight has it explained well, as usual. A few points of addition.
First, the average Japanese does not have the language ability to operate on his or her own, especially men. One of my American business acquaintances told me his survival tip, which was when he would ask strangers for directions, he’d chose younger women instead of younger men, since the younger women were more likely to speak some English. It may just be that he likes talking to young women, but there is truth spoken.
A few years back, the used to talk a lot about “Narita Divorces” where women would come back from their honeymoons abroad and break up with their husbands at the airport. One of the reasons was how poorly their husbands performed in foreigner counties. (And stop your snickering, this isn’t about their bedtime behaviors.) Since younger women are much more likely to have gone abroad, they’re more experienced than the men, and will have been abroad several times previously.
In addition to the language issues, there are several factors for the bus experiences.
Japanese have fairly rigid rules in society, and tend to have anxiety if they don’t understand what the rules are. There are more formalized intermediaries in Japanese society, so having coordinators and intermediaries handle arrangements happens within Japan as well. The expectation of the happening in a foreign country is an extension of their cultural expectations.
However, perhaps the biggest reason that you see a lot of Japanese tourists in busses throughout the world is that is how many Japanese take vacations in Japan as well. Japanese often like to be spoiled on their vacations, and have someone else so all the arrangements, etc. They don’t seem to mind being managed, and where a lot of Westerners (including myself) would feel they are being treated as children, they enjoy the “groupness” of it all.
I think there is more of a tendency of Western people to see going abroad as an adventure, so sharing the experience with 50 people in a yellow bus takes away from the uniqueness of it, where Japanese often prefer to be with others.
(Obviously we’re talking stereotypes here, and there are good numbers of solo travelers as well.)