I think it's less calculated than that.
Most startups fail. That's scary. The natural response to fear is to run harder, run faster. And when the ride isn't scary, it's really exciting. Who can sleep when the growth curve looks like that?
And all this trickles down to the employees. And up to the VCs, the dumber fraction of which will try to measure progress by hours spent. If you aren't sweating, you aren't working, right?
Honestly, I think there's little economic benefit to all those hours put in. Especially for people writing code, where I think it's incredibly easy for tired people to be negatively productive. It's fear, excitement, and macho bullshit, not rational self-interest at all.