The primary driving force for U.S. members of congress (MoC) is pleasing your constituents and pleasing your donors; with pleasing your party's national convention being a third priority. Despite many of the other answers, an American MoC doesn't vote in an attempt to preserve a uniparty. If a MoC represents a district heavily predicated on sugar, and they receive large donations from the oil lobby, then that MoC's voting record is going to reflect a lot of pro-sugar, pro-oil bent. They might vote nay on anything outside of pro-sugar, pro-oil interests unless they can get a small pro-sugar or pro-oil earmark added to the bill or curry a favor that they can leverage later. There are a lot of MoC who do not have enough spine to vote in a way that would be politically unpopular in their district; i.e. pleasing the district voters is paramount. This is mostly out of self-interest; but anyone who believes in Republican representation respects the importance of representing your constituency's values, tempered with your own greater judgment (you were selected to be the smartest version of your constituency, after all).
However, many politicians actually do have ideology, and they got into politics for a reason. Outside of the tactics they use to get elected and stay elected, they still have an ambition to set policy on some economic and/or social issues that they care about. This is the battleground of most congressional votes. Most bills connect with each MoC in some ideological way, either for or against, but fail to connect with the special interests that they are personally subject to. Apart from the MoC's ideology, each bill tends to have some implication to the party platforms, and so all other things being equal, a MoC is likely to vote in according with their party platform. For instance, the DNC's platform favors greater funding for public education, so if a bill came up that appropriated greater funding for public education, it is likely that almost all Democratic MoC would vote in favor of it, excluding possibly 1 or 2 who might not be able to due to their district's special interests.
There are very few times or motivations when it mutually benefits both parties to collude to pick election winners. Party polarization actually drives greater campaign donations and richer politicians, so collusion would generally not benefit them on the whole. Moderation would occur either as a tactic to seize moderate voters and win elections, or out of the altruistic recognition that moderate policies are the most tolerable for the greatest amount of the population.
There are, however, a small number of policies in which the two parties do benefit from cooperating on. For instance, any radical campaign finance reform would strengthen independent parties, weaken Democrats, and weaken Republicans even more. Also, replacing the first-past-the-post (FPTP) election system with a superior system would strengthen independent parties while hurting the dominance of the top two parties. More inclusive public debates would strengthen independent parties while hurting the dominance of the two top parties (they would also reduce the public's ability to decide between the Republican and Democratic candidates, which is actually problematic, given that some voters already struggle to pick the better candidate). These are all areas in which the top two parties are unlikely to vote for reform.
But anyways, to the original question: why WOULDN'T they collude? The answer is because there is nothing stopping an outsider like me from running in a party primary, trying to get the party nomination, possibly winning it, and then trying to win the general. Even if the RNC offered to concede a loss in Ohio in exchange for a win in Illinois; they can't stop me, a self-interested party, from trying to run and pursue my own interests, which are in direct conflict with their agreement. Another reason is because there is genuine animus between the parties. A third reason is because there are not any real amount of benefits to this collusion -- why compromise when you can attempt to win it all? A fourth reason is the risk of facing charges of election tampering or conspiracy to defraud the public if they are caught. I honestly don't see how a rational mind could see there being more benefits than liabilities to party collusion (at least in America's current political environment. I could definitely see it in a system where several strong parties are all legitimately in-play).