But the limiting factor isn't whether you're in the airport, it's whether you can arrive at the airport with that much a time margin reliably. It does not take many traffic accidents or an unexpected rail disruption to mean you miss your flight, for example - the 2 hour window is essentially to force people to account for unexpected delays.
Having faster flights could also increase the frequency of such flights, which lowers the impact of a missed flight, which lets people be more loose with getting there early enough.
