The question is why and how the politics plays into this. Why does NASA and other space agencies usually not (if ever) complain about their minuscule budgets? I can only assume that doing so would be politically foolish. If NASA were to complain about their small budget---which probably seems very high to the general public and probably to the typical politician too---maybe they would seem careless, greedy or incompetent, and get even less support. Is the smart thing to do politically to never talk about failures and problems, and only talk about what positive results are being achieved with what you have the way to go? To be optimistic even when the situation almost couldn't be worse? Is that the reason why they are doing this?
Since this is such an important issue, I wouldn't be surprised if there has been conducted scientific studies on how to manage the public and the politicians regarding the budget. If there are any such literature it would be a great part to include in any good answer.
Some simple justification for budget issues (adjusted down from a lengthy mostly hand-waving rant, and toned down):
For NASA, the budget was around 4.4% of the federal budget  during the Apollo days, whereas it is now around 0.5% . It also appears that there do exist resources, but they are allocated for other projects, e.g. the F-35 program at approximately $1,5 trillion . ESA and RFSA both have a fraction of what NASA presently has, and doesn't appear to ever have had big Apollo-like budgets , although RFSA certainly had more resources at its disposal during the space race.