Part of the reason is that the framers of the US constitution hated parties, and seemed to take a "see no evil, hear no evil, speak no evil" approach. With no b

Part of the reason is that the framers of the US constitution hated parties, and seemed to take a "see no evil, hear no evil, speak no evil" approach. With no backbone of written law to fall back on, US political parties only rely on their own rules as an organization of people. All the problems you described do affect them, and they used to be even more exclusive and opaque. The candidacies of Donald Trump and Bernie Sanders are fruits of the opening up of parties, effectively allowing takeovers by determined blocks of people.