As others have said earlier, there is an overlap in quality between the average MIT/Stanford graduate and strong graduates from state flagships. From both places, many of them will end up at solid places to work like Google, Microsoft, Apple, Amazon, Facebook, etc. Likewise, a mid-level state grads and a lower level MIT/Stanford grads will end up at many respectable companies throughout the world. And from the limited perspective of looking within one company, it does indeed appear that the people from Stanford and MIT sit the same little cubicles, doing the same kind of work…
What a person working in a normal company doesn’t see are the MIT/Stanford grads that ended up at the most selective companies (companies more selective than those I listed above). And these highly selective companies tend to recruit the most talented graduates from highly selective colleges. It is not because it is impossible to find similar levels of talent in a state flagship, but because the concentration is much higher at a highly selective school which makes recruiting much easier.