That's not the US system. It is definitely something abused within the US system but to call it the norm is ludicrous: realistically most families can't buy the university a new building (such a "buy in" also is not particularly unique to the US nor uncommon in rich, middle, or low income countries). Most people get in based on merits, but merits are fuzzy and incredibly difficult to measure. All measures of merits can be exploited and hacked as measures are only ever a guide(see Goodhart). Pure meritocracy doesn't exist because it cannot exist. You don't get closer to that meritocratic goal by setting up more bottlenecks or strict standards, but rather by recognizing the chaos and messiness of the real world: that measurements come with noise. You should have measurement, but it needs to be deeply ingrained that measurements are noisy.