Essentially , the DNC and RNC have at this point successfully executed regulatory capture of the US political system. That is, it’s incredibly difficult to run for anything and win at a state level or higher unless you’re affiliated with one of those two, and the parties are able to exert a substantial degree of control over any and all candidates affiliated with them simply by giving or not giving them financial and political support. And the FEC has rules that effectively entrench both parties - the barrier to entry for 3rd parties is obnoxiously high, and the vagaries of our FPTP system (and for president, the electoral college too) mean that we’re effectively forced to vote tactically if we want our vote to mean anything in general elections.