How and why someone will work 80 hours a week? Don't they have families, friends, sport, or a hobby? Why is it necessary?

In startups that's usually how they trick you: they hire you even if you don't have much experience, they give you a decent salary, then they make you feel like you have to work more in order to "prove" their choice was the right one.

I've also seen this "culture" of "we're one big family" in startups, which just feels weird and forced.

One startup I interviewed at, as an experienced developer, told me (paraphrasing here) "you'll be paid a salary lower than what you probably expect, and your first few months you'll need to work overtime to get used to our trendy tech stack, we don't offer health insurance but we do offer...not quite shares, but, sort of ownership interests. You could become a millionaire!"

Yeah no thanks.

Because of OKRs. Management is setting it so that no-one can reach the objectives. Then there is some speech that OKRs are not really meant to be completed fully but they will still hold it against you if it's not 100%

No, because these kinds of places suck the blood out of people in their early 20s. Usually after they have moved to a new area and left behind all their social connections.

When you're 25 and have no ties and no responsibilities, working 12-15 hours a day isn't the impossibility that it is when you are 35 and have a wife, kids, your own house, and social obligations.