For many modern (knowledge work) jobs, it's volume of tasks, not duration, that seems to induce burnout.
For instance, if you have 1 central task for the week--say, write a report--but there are ten subtasks (hold 5 meetings to prepare, read 3 background papers, ...), and then each of those have a bunch of subtasks (Slack each 5 meeting attendees 10 times to coordinate and schedule, get interrupted when reading each paper so have to resume X times each) ... this 1 central task can easily be dozens or hundreds of subtasks.
And then each coworker is doing the same thing, adding tasks to your load (you have to read their messages, respond to their emails, etc) in a multiplicative way. Newport calls it the "Hive Mind" in one of his books. The number of total tasks, from very small to very large, each individual has to accomplish in a week ends up far far greater than expected on the surface. And adding to that, they come in in an unpredictable way.
All this adds up to burnout, not just the number of hours. It's the intensity, and the unpredictability.
I've experienced this myself. I was at one of the FAANGs, constantly bombarded with new tasks, and felt burnt out. Now, I'm at a startup--very much inspired by Cal Newport, and using AI and context awareness to make teams operate together more effectively (see bio)--and I'm working far more hours per week than before, but with less interruptions and less distraction, I'm able to focus and feel far less burnout despite the increased hours.
All this to say, we really need to rethink how we work, not just how much.