It is indeed partially true that doing high level work simply requires extended hours and unusual dedication to achieve anything. However, that is not what is primarily at work here.
The main factor is a sort of culturally normative and ingrained rite of passage / hazing ritual / bullying / dominance effect, which primarily serves to inflate the boss professor's ego at the expense of the grad students. Getting a PhD was difficult and grueling for the professor, so they are damn sure going to make sure it's difficult and grueling for their own students.
The only way they themselves got through it was internalizing the attitude that a grueling work and study schedule is simply normal and simply the price of success.
This exercise of extreme and essentially arbitrary power over how the students live their lives, far beyond what any normal job could remotely require, is very gratifying to many professors. They've worked so hard, suffered themselves, and now THEY have this absurd power over others.. It makes them feel important and powerful at a very primal level to tell their students they have to live under these extraordinary conditions, and then see them obey.
They can literally choose whose career will live and whose will die, whose dreams will happen and whose will be broken and swept away without a thought.
To fix this, the professors need to have a lot less arbitrary power over their students. There needs to be another route to a PhD besides enduring poverty and years of ritual self-humiliation and long form ass-kissing. Only then will the culture shift, as the "grind mode" professors are replaced by new professors who didn't have to grind.