Because the fact that daily work exhausts people and drains their energy is some kind of complex nuanced issue that requires dedicated research to verify?

Obviously it depends on how demanding/time consuming the daily job is, but that's a standard observation with millennia of empirical evidence.

I swear, it's like people can't even tie their shoes anymore unless there's a long-term study (and preferably, meta studies) to prove them that tying one's shoelaces is good.

It isn't a "complex nuanced issue", talking about the depletion of some non-existent entity (creative energies? srsly?) is at best a failure of self-expression and at worst spiritual nonsense.

I guess people get defensive because of this very frustration of being wrong and unable to correct themselves at the same time.

It's not creativity, it's mental energy. Most people don't have the capacity to even work 40 mentally demanding hours per week, let alone double that. Just look around at your colleagues and ask how many are really working hard eight hours every day for five days straight. I know I'm not. My best output comes in shorter stretches of flow a maximum of a few hours long, and often not even every day.