I'm very involved in party politics and have run for office.
The Democratic Party isn't a single thing. I can't even summarize it briefly. One facet is a bunch of independent groups sharing the same logo, tribal identity. Another is a hierarchy of squabbling ad hoc volunteer organizations fighting each other for relevancy. A third are the Democratic themed campaigns which are largely parasitic on the party organizations. A fifth are the loose coalitions of like minded interest groups also squabbling for resources and relevance.
The DNC isn't even that influential. Closest analog I can think of is a trade association that hosts conferences.
I can't speak directly to the GOP's structure. From observations and contact with their players, they're similar.
People don't get how candidates are chosen to run for office. Especially at the top of the ticket. I'm talking before the primary / caucus. Special interest groups form and then go candidate shopping. "Superstar" electable candidates, like GWB and Obama, are selected, groomed, promoted to other interest groups (investment clubs, individuals). Note that these groups filled the vacuum left by the decline of the traditional GOP and Dem party machines.
All the other candidates are delusional self promoters. They literally have almost no chance. But they run anyway, to promote their brands, to take one for the team, to get the experience, because anything can happen during the campaign cycle, etc.
Me personally, I'm still gobsmacked that Trump iced out both Cruz and Rubio. My initial post-mortem for the GOP 2016 is Trump's dark horse victory just demonstrates how dysfunctional the GOP has become (the Tea Party has driven away their "moderates").
A hundred years ago, it probably seemed possible to build a third path - but with our modern understanding of game theory, we know a two-party system is the inevitable result of first-past-the-post. Building a third path is impossible.
This creates an inconvenient contradiction - the moral legitimacy of the person sitting on the bench derives from the fact that everyone has the same right to sit on it, but we can quite plainly see the criteria for moral legitimacy are not met in practice.
Some people would say we should aspire to meet the criteria for moral legitimacy; and therefore that, regardless of their private status, their internal elections for access to the path should be free, fair and open to all.
Of course, other opinions are available. Some people would argue that the private property owners' rights were more important, morally, than the legitimacy of the presidency. Or that the idea of pursuing morality in politics is a joke. Or that our consent to our rulers isn't derived from the moral legitimacy of their being fairly elected, but comes from some other source such as habit or force of arms. Or that everything is a shade of grey, america is better than russia or north korea, and improving sounds like a hassle who can be bothered?