It's the same reason that federally funded scientific research is only available in for-profit journals. Someone has to pay for the costs associated with publishing things, and the scientists/FCC don't want it coming out of their budget.
Of course, the money being paid to access papers in for-profit journals is also coming from the government, just in a less efficient way.
The cost of publishing - especially written documents - has dropped to essentially free in the last two decades, though. I hear your argument from time to time, but it's a really hard sell these days.
Of course, the money being paid to access papers in for-profit journals is also coming from the government, just in a less efficient way.