Employees love hybrid work, so why are so many burning out and quitting?

Because work still sucks?

In spite of being able to work from home, I'm still working on pointless nonsense that is being mismanaged and likely to never become a real product but instead peter out and die due to lack of vision and follow-through and constantly changing priorities, my wages are not rising with inflation, my career is not being developed as I was promised and I'm stuck doing heavy lifting that I was assured was temporary? And on top of all this, in the middle of a virus spike, my management chain is giving me some line about some theoretical synergistic boost from working in the office and pushing to end WFH, because they'd rather be wardens in the panopticon then actually responsibly manage staff and a project?

I presume the situation is comparable in a lot of places, and I am entirely unsurprised that people are quitting.

Better jobs come along.