If the founders are working 7 days a week, the first employees are excited about working 7 days a week, then it's kind of weird to hire a new person who isn't. Most likely, everyone will just quickly be thinking, hey the new guy sucks. Just gets everything done slower than everyone else.
At some point you have to, though. Anecdotally it seems like at some point the founding team gets tired and the CEO makes a call, okay it's time to go down to 6 days. And then later 5 days.
Different companies work different ways. I personally wouldn't want to work 7 days a week at this stage in my life. Or 6 days a week. Weekends with my kids are too important to me. But when I was younger and just breaking into the tech industry I would have done it. So, find a company that fits what you want. It's okay if not every company is a fit for you.