I think that it is because they are exercises in Negative Mimetic Desire.
Mimetic Desire is a term I got from Rene Girard and refers to the pattern of wanting what other people want. To go further, in most situations most of us do not know what we want until we observe other people wanting it. We imitate other people's desires.
Negative mimetic desire is wanting what some particular other people reject, because they reject it.
In the very complicated world of public policy and federal government financing, most of us do not have a clue about the merits of one proposal over another. We depend on signals from opinion leaders we trust that this proposal is good and that one is bad. We end up wanting as non-negotiable what the others have said they cannot accept.
During the health care debate, the public option became a non-negotiable symbol, when most of us had not heard of it before. The Individual Mandate was a GOP policy preference up until the point that it became a crucial part of the Health Care bill that Obama wanted. What Obama wants must be bad = negative mimetic desire. The clearest example was Joe Lieberman's quick flip-flop on the proposal to let people over 55 buy into Medicare. At first he liked it because it was an alternative to the Public Option (which was to be opposed because the left wanted it), but within a week, he moved to oppose the 55-Medicare buy-in, because as Lieberman stated explicitly, the proposal was making too many liberals happy.
Of course, in the legislative process leaders signal one set of priorities in public and then proceed to compromise them in negotiations. As a result, followers are confused and feel betrayed. Impatience with legislation grows.
It used to be that Congress passed individual authorization and appropriation bills, for each part of the government. Then, they were consolidated into a single budget resolutions. Now there are these deal last minute deals under the deadline of either government shutdown or default. The deals get bigger and the negotiations get more symbolic and abstract and the public gets more and more polarized as one set after another of supposedly non-negotiable positions get bandied about. Everything becomes winning and losing. Eventually the leaders compromise, but the rest of us don't.