To expand on the point above some more. If there's a deal to be made, generally politicians will make it - this is where my game theorist checks in and mentions "Pareto optimality", as if by trading things they consider irrelevant they can get something they do care about passed, they'll likely do it. It's just that, at least in America, probably the biggest concern for Congressmen are local issues like bringing federal funds into their area and protecting local businesses, which can often clash with national priorities everyone pays like balancing the budget.
Also, I'm not sure that the USA is even the worst off in this regard (and comparing it to other 1st world countries). I'm going to propose that the amount of "pork" in your average bill is related to two things: the strength of party discipline and the number of factions at the table. Party discipline allows you to ignore the whiners in your own party who disagree and would normally need to be bought, and the fewer the parties the easier it is to pass legislation (passing it with just one party with a simple majority is ideal). The United States, with only two political parties, is actually pretty well set up in this regard- it's just that party discipline is weak, with a number of "defections" on any random vote fully expected. Countries with proportional representation systems have more of a problem here, as each party can demand their "cut" in a bill. The worst example by far is Israel - the Shas party of ultra-orthodox are total mercenaries on economic issues, only asking that they get to run the religious establishment of Israel. Since they can often make the winning coalition, that's how Israel has a lot of dumb laws like the ultra-orthodox getting to set the rules for all marriages, basically forcing secular Jews to head to Cyprus to get married. This is a case where blatantly unrelated items really do get nailed together into the same deal (whether it's set up as an amendment/rider or not) to really bad effect. Another bad example might be Italy, where Supreme Jerk Berlusconni gifted the Left with eternal infighting by passing a law that helped splinter it into a zillion tiny infighting parties. The last coalition government the Left held under Prodi was really unstable as a result and had to do things like hand out the Minister of Justice role to corrupt Mafia-affiliated conservative centrists and other positions to loony Communists.
Interestingly enough, by my theory above, the UK should be the most pork-free: Two and a half party system with strong party discipline. Labour single-handedly rules, ignoring the Tories. The Lib Dems are basically more liberal than Labour and not after something totally different, as well (unlike nationalist/ single-issue parties that sometimes crop up when there are too many parties a la the Bloc Quebecois / Shas / various pensioners parties for old fogies). On the other hand, my understanding is that Blair's government engaged in some wholly irresponsible porky giveaways to placate people before elections, so maybe I should just say "could" be the most pork-free. Of course the most perfectly pork-free government would be a dictatorship... since "pork" is to some degree what the people want but really shouldn't get, having a government that didn't have to listen to the people could accomplish that the best. Of course the cost of ignoring the people is... horrible. So yeah, pork is bad, but going too far the other direction wouldn't be better.