Kia Jarmon, a leadership consultant who has worked for nonprofits and now consults with them to improve operations, told me that it's because having purpose on the job didn't solve a lack of purpose off the job.
Companies need to allow workers time to disconnect from their jobs and support them with basic tools that make their working lives manageable — from flexibility to good pay to interpersonal culture. Simply having a job that helps the world doesn't make the stress and problems of the workplace go away.
I asked her whether purpose could be burnout's kryptonite, and she said that purpose addressed only the values or meaning aspect of burnout. Burnout is often a combination of mismatched factors.