People in their early 20s are also much less likely to have other responsibilities "intruding" into their headspace. It's a lot easier to be monomaniacal when you don't (for example) have kids yet.
Also, Hans Albert Einstein was born during Einstein's miracle year.
This was in an era when fathers had little to do with childcare. I don’t know about Einstein’s specific situation, but even 40 years ago almost half of fathers had never changed a diaper.