Because we rarely have the option to free up time. Most of us work some form of a full-time 9-5 job, with a specific amount of days off available because that'

Because we rarely have the option to free up time. Most of us work some form of a full-time 9-5 job, with a specific amount of days off available because that's how employers prefer it. Knowing that the rest of your pre-retirement years will be spent in such a configuration you'd want to spend additional time initially on getting better, advancing your career etc., because if you can't do anything about the amount of your free time, at least you can influence its quality by earning more. I for one would be happy to work part-time, but the lowest engagement I ever managed to negotiate was 90%. Best I've seen was 80%. 50% is only ever available for short-term gigs.