Meanwhile our political system is so not rigged, that everyone sort of knew, who would be president in next elections for like 20 years.
Yours is rigged in a broad way: it's obvious that one of the two parties will get their candidate to represent the country, since money decides the outcome when the parties' lies are of roughly equal size. It's also obvious that the two parties will divide most of the political power between them.
However, the uncertainty about the exact balance of power and the free-mass-media-dependent chaotic fluctuations in approval keep your politicians constantly on their toes, not simply trying to stay on the electorate's good side - every politician does that, except for the most psychotic of dictators - but trying to do it
better than the other party.
That kind of an ass-kissing competition keeps the parties busy watching that their members don't screw up in their duties too hard, weeding out those embezzlers that are stupid or greedy enough to get caught, and making sure the system is running smoothly so they can juggle around some numbers and claim sole responsibility for it in the next elections.
If the parties were secure in their knowledge that they'll win the next election, they'd stop pretending to look out for the interests of the people and devote all of their energy to siphoning the money out of the country. Which, in turn, would hurt the economics and so on. The list of historical cases where democracy went wrong because of a lack of competition would be enormous.
Your system sucks. It's corrupt. It's oppressive. But - your system sucks less, is less corrupt and less oppressive than all the other regimes we've seen so far. It's so much less corrupt than non-competitive systems that Americans seem to believe the word "democracy" makes everything better somehow, and even managed to delude the rest of the world into thinking the same.