As someone who was a part of the hustle culture at a previous company, I think it's largely because you don't want to reconcile that all the sweat, blood, tears, etc you've burned on the product weren't worth it. It HAS to be worth it, and if you don't believe it then you're the problem, not the company (oof). There was also the fear of not being seen as a team player and being seen as "less than" if you begin to slow your pace.
That's at least how I used to rationalize it to myself. Was verrrrry wrong and was much happier and healthier once I jumped ship.