I don't think we can so easily pin it on capitalism. Capitalism brings incentives that drive work hours and expectations up for sure, but that's not the only thing in play.

Workers are often looking to make more money, take more responsibility, or build some kind of name or reputation for themselves. There's absolutely nothing wrong with that, but that goal also incentivizes to work harder and longer.

There's no one size fits all description for workers, everyone's different. The same is true for the whole system though, it doesn't roll up to any one cause.

Unions had early wins that mostly either didn't go anywhere, or the companies worked around. The real win that normalized it was for capitalistic reasons, when Henry Ford shortened the workday/week because he wanted his workers to buy (and have reason to buy) his cars. Combined with other changes, he figured he'd retain workers better and reduce mistakes from fatigue, and when he remained competitive others followed suit.