I've noticed how government jobs in the US are known to have generous pensions, healthcare plans, vacation time, etc. as compared to the corresponding job in the private sector. (I haven't actually done any comparisons specifically, but it's something that many people say, so I am assuming that this belief is true.) Why is this the case?

Someone may answer with, "This is because government workers have unions, which push for better benefits." But why can't the industry employees form unions as well, and thus receive equivalent benefits?