We have two organizations that peddle in power. The GOP and The DNC. They are private "non-profit" companies. They have employees and everything and they are free to do whatever is needed to push the candidate they think will best serve their needs first. They both sell that power to clients. These days those clients are more direct and they collect through campaign donations, job guarantees, speaking fees, consulting contracts through families, trade deals, stock tips, family opportunities, Since Citizen's United PAC money, and sometimes but rarely nowadays direct pay offs. The corruption is right in the open the difference between here and elsewhere is its all perfectly legal.

I actually come from a country with a mathematically rigged voting system (not quite as much as the US, but still the current guys in power got 41% of votes and have an absolute parliamentary majority with 52% of parliamentary representatives) but lived for almost a decade in The Netherlands (which has Proportional Vote) as well as about the same in the UK (which is more like the US in that regard than the rest of Europe) and my impression is that there are 2 things pushing that dynamic in countries with such rigged voting systems vs the ones with Proportional Vote like The Netherlands:

• People do a lot of tactical voting in FPTP and similar because they can't find electable parties whose combination of ideas of how the country and society should be managed aligns mostly with theirs, so they vote for a "lesser evil" and often driven by "kicking the bad guys out" rather than "bring the good guys in". This makes it seem like the parties of the de facto power duopoly are more representative than they really are - in a PV system they wouldn't get anywhere as many votes because even people with niche takes on politics would find viable representation in parties with a much more similar take so wouldn't vote for them and would in fact be more likelly to vote positivelly rather than negativelly.
• The press itself in countries with the representative allocation systems rigged for power duopoly tends to present most subjects as having two sides only. This is complete total bollocks: people are complicated, social systems are complicated and almost no social/economic subject out there is so simple that there are only two reasonable ways of handling it and no more than two. This kind trains the public to look at things as two sided, reinforcing the idea that the system is representative as well as the us-vs-them mindless tribalism and even bipartisanism rather than the politics of consensus building.