10k hours has a lot more to due with human lifespans than it does anything else. 4h a day 5 days a week 48 weeks a year for 60 years is only 57,600 hours. Sure, after someone has been doing something for 10 years they hit diminishing returns. But that's in context of a relatively low lifetime limit.

If people lived 1,000+ years people would be talking about 100,000 hours.