The thing is - in startups, there always is an external deadline (assuming you're not cash flow positive or profitable) - it is tied directly to your runway and when you'd run out of cash. That is the ultimate deadline for pretty much all startups.
The very early stages of a startup (and even later stages) usually require this much effort because you honestly don't have enough money to last you the "right" amount of time you need - and therefore you need to put in more time.
As you said - this isn't sustainable long-term - but, especially in the early stages, you may not have a "long-term" in that venture if you're not able to put these hours in.
Not to say it's impossible to find a great work-life balance for an early stage startup - but it's highly unlikely.
The very early stages of a startup (and even later stages) usually require this much effort because you honestly don't have enough money to last you the "right" amount of time you need - and therefore you need to put in more time.
As you said - this isn't sustainable long-term - but, especially in the early stages, you may not have a "long-term" in that venture if you're not able to put these hours in.
Not to say it's impossible to find a great work-life balance for an early stage startup - but it's highly unlikely.