It's called the race to the bottom. People see someone else getting something they don't get and instead of saying, "Golly gee, how can I get that?", they say "That person doesn't deserve it." (and what they're really thinking is "I deserve it just as much as they do, if not more, so if I don't get it they shouldn't either")
Call it envy, jealousy, bitterness, whatever. It's been around since the beginning of time.
Part of the reason unions tend to get more than individuals is because of bargaining power. In short, the average joe has none. But if the average joe gets together with a bunch of his pals, together they now have better bargaining power. But if you're an individual who can't figure out how to unite with other individuals, you're likely going to be pretty bitter when you see other people doing what you wish you could be doing.
The race to the bottom is what happens when bitter/envious/jealous/whatever people want to take away what other people have that they don't have. Instead of taking the positive approach of trying to get where the other people are, they take the negative approach and try to bring the other people down to their level. In short, these people are an employer's dream. Whoever can undercut their fellow employees fastest "wins" (and usually these people don't have the mental wherewithal to realize how Pyrrhic their "victory" really is).