People start their careers in their 20s and retire at the ripe old age of around 60. Why do we chose to have just one career in our entire lifetime?

Do we really need to stay in one career for decades to learn about a professional field? I think I can learn about a field in a couple of years and become an expert 5. I think I can work in that field for another 5 years and then what? I would want to move on to a new field.

How about we redefine the duration of the careers to 10 years instead of the Industrial age 30 year career?