Because mixing units but keeping coefficients 1:1 means there's no math, as long as you measure your bodyweight in pounds and have a way of measuring your protein in grams (which food packaging does, even in the U.S.).

Mixing coefficients instead of units means you need to express it as 2.2g/kg which means having to do a calculation, or two calculations if you need to convert your bodyweight from pounds to kilograms.

Can you explain what you think is confusing about 1g per 1lb?

I suspect you're cargo culting "never mix units!" without realizing that the advice is talking about denominating the same thing in different units (thus having to do conversion).

I'd be curious to see your explanation of the mind bending and error prone nature of "ok, I weigh 200lbs and now I will ensure I get 200g of protein today" and the exact scenario where you think it would pose a problem. Frankly I think you'd realize it doesn't exist.

It’s a 1:1 ratio with those units. Hard to imagine anything less mind-bending/error-prone. Set your body scale to pounds and your food scale to grams. Done.

Because (at least in the US) we weigh ourselves in pounds and weigh protein by the gram. That’s kinda of the long and the short of it lol.

I'm from Canada we mix both units all the time, or maybe its just me. But I think this has to do with most foods being labeled with grams, its easier to add in decimal. But most people weigh themselves in lbs.

It’s just a common heuristic. It’s not like it’s used for much other than a staring point for how much protein to consume.