> But what might be causing the decline in T?

The things that T contributes to becoming less important to survival, social status, and economic status probably are all factors. There's a number or candidate physical environmental factors, too, IIRC.

Not to go all MRA here, but nobody cares. Nobody's going to bother with the research and meta-analyses required to get to the bottom of the matter. Our health concerns are of no concern to this society.

And while many states are legalizing marijuana, it's still a life-ending felony in all 50 to get caught with a vial of Testosterone Enanthante in one's gym bag.

Even if you have a valid prescription, e.g. for ART / TRT?

I'm not going to raise the MR issue, but when you say "nobody cares" it seems you're definitely hyperbolizing a bit.

Sure, but if one of them is more easily controllable maybe more research in the direction would provide some low hanging fruits.