It's a complex issue.
Not everyone knows what their best is. What some people think is their best has much or little to do with the expections for best. People who are burned out don't feel they have enough energy to do their best. Some feel they're already doing more than they signed up for or are being compensated for. People who don't feel connected, valued or free don't feel motivated to do their best. People more focused on constraints than progress don't engage their best.
If given the choice to do their best, many people would choose that over alternatives. Each would have their own reasons for the choice. In the most sustainable examples, people make the choice because of how it makes them feel valued, connected and free in their work.