Sudoko and crosswords can also be mentally strenuous. As far as I know nobody gets awarded a PhD for solving puzzles.
Huh?
Is there something more specific that you think theology has contributed to human knowledge?
The issue here is that one can study God their whole lives and only yield personal benefit. That does fall under the category of "human knowledge" but I assume you are speaking about knowledge that everybody can use. Theology isn't about linear discovery, that's what Science is for. If 100 people follow 100 spiriutal paths you get a pile of "human knowledge", but it only benefits the individuals. It's sort of encrypted within the individual human experience.
A pitfall of religion is that they take a spiritual path tread by a prophet and then try to impart that experience onto others. It's been shown to not necessarily work which is why they often have to develop systems of enforcement which just puts them on the path of desolation. I think the human experiment of religion has taught us that spirituality is a deeply personal pursuit. I am not saying that we shouldn't share and compare notes, but it's going to be difficult to translate.
Theology teaches us how we can rely on God for strength. It teaches us that the universe is not a finite place and that things happen for a reason.