The problem is that the two parties capture huge swaths of people based on issues that are important to that person, not to the party. There is no real unified platform from either of the two major parties, just look at this (rough, off the cuff) list of things they care about enough to capture:
guns OR god OR small government OR military expansion OR anti-abortion OR anti-immigration
VS
anti-guns OR (a)religious tolerance OR social service OR globalism via economy OR pro-choice OR pro-immigration
Both of those 'platforms' are internally inconsistent (i.e. how can you have a small government that also has a giant military and regulates trivial stuff like what happens in your bedroom) and are simply designed to cleave off people that give huge amounts of fucks about a single issue. What if you're pro-gun and pro-social services? Well, pick which one you care about more and vote for that party...