**Losing focus after a long time on the same thing could be the brain becoming aware of the opportunity cost of focusing on only one thing. **
You may not be limited by physiology when it comes to the duration you can focus on a thing, but instead your brain starts to resist because it starts to think “am I still focusing on this thing? Aren’t there other things that might be more important?
If you’re focusing only on A, you might miss B or C that could save your life or allow you to have more children. Thus we evolved to slowly lose interest in things.