Let's take a more objective look at the number of authors per papers. I selected ACM Multimedia, one of the most famous multimedia conferences. The 20th Annual conference was held in last October, so there is a good time span to observe trends.
You can see that the number of authors per paper has increased quite fast, in the past 5 to ten years. The trend is the same for other multimedia conferences such as ICME. Here are the main reasons for the increase.
1. Just like you need publications to graduate, those above you need publications for different purposes. In some universities, professors get a pay cut if they fail to publish a given number of publications. Non-tenured faculty need papers to get tenure. Postdocs need more papers so that they have a better chance of getting in to the faculty. Funding in academic departments are allocated by looking at publication records. I guess you now know why everyone above you want you to publish.
2. Things at research institutes are worse. Most researchers are on contracts nowadays, and they have to publish to keep their jobs. Some institutes (example: I2R, Singapore) require researchers to publish only in conferences recommended by them. You might feel very happy that you got an internship at that famous research lab, but a greedy-eyed researcher might be waiting there, wanting you to publish from day one.
Okay, people needed publications not just 10 years ago, but before that as well. So what has changed? Funding for research has reduced quite rapidly during the last decade. If the funding increased by the numbers, then the number of people who used the funding has increased. So, everybody wants more papers with less money. The solution: put more names in one paper!
One young researcher with whom I work for a short while told me "if we put each other' name in our papers, we both will have publication records that are twice as long." Sounds mathematically correct, but ethically, wrong. When I pointed that out, he responded "I did not create the system, I am just trying to survive in it."
3. In the early days, researchers got funding from "funding agencies", and mentioned the agencies in the "AcknowldegmentS" section of the paper. But since recently, some research institutes are using money to "buy" papers. When you get funding from such research labs, they assign you one or two collaborators, who might (or might not even) have an occasional Skype meeting with you. You put the names of these collaborators on the papers. Arguably, it is a good thing to have a collaborator from a large research lab. But it is not correct to put names on other people's papers just because of money.
What these people don't realize is that the industry is watching. Very soon, there will be many researchers with huge publication records (okay, there already are). Once credibility on the publication system is lost, it will be very hard to restore.
So, whose names should be on papers? Ideally, the names of thoe who did the work. The supervisor's name is fine, because he/she guides a student who is learning to do research. Proofreading, being a friend, providing money, etc., do not count as valid reasons.
But then, you as a grad can't do much about the names your supervisor inserts in your paper. At least try to put your name as the first author, if you did the work. In many universities, the professors are not allowed to be first authors when students write papers. And when you graduate, be more ethical if you still publish.