Why only 1 cabin bag in US?
 

Why does US (Not sure which authority) limit hand luggage to one piece. I am particularly curious, as it seems, when flying in the US, many people have more than one anyways. (A massive rollaboard plus a backpack plus whatever) If they really don't care, why not allow carriers to permit pax to have more than one?

What carrier are you talking about? I have never encountered that. At least the legacy carriers such as UA, AA, and DL all permit two.

Specifics would be helpful rather than a blanket statement such as "why does the US...."

It is one bag plus one "personal item" - small backpack, laptop bag, briefcase, handbag.

Which pretty much means 2 bags unless you go crazy.

I have on occasion seen someone carrying on two 21" rollers. Definitely against the spirit of the second item being a "personal item", but they were not challenged by either gate agents or cabin crew.

Some EU airlines - notably Air France - will allow 2 carry-on bags *and* the 'personal item' in business/first class.

But for flights departing in the U.S. they have to limit it to 1 carry-on plus 'personal item' and advise every passenger that they'll have to check the 2nd carry-on.

There are some exceptions, like camera bag and diaper bag. I have also, on occasion, physically carried on two suitcases when traveling with my family. Obviously, one belonged to a family member (i.e., my wife).

Don't most? Most in my experience do. I feel like even Air Canada did previously. (But could be wrong) And seems like US has some official rule against it, which is confusing to me given that it seems like personal items in US are merely second cabin bags.

If I'm traveling with someone else, one of us may be holding some of the other person's bags. In other words, someone with two roller bags may be carrying both on behalf of one of their travel companions.

Generally I find Americans carry on far more than Europeans and, whatever the official rules are, US airlines are much more lenient on the issue. IMHO this is one of the principal reasons why air travel in the US is significantly more unpleasant.

My European colleagues do tend to check luggage on <5-day trips, something which few Americans tend to do.

I have no idea why this is, but I do find that with Pre-Check and the ability to arrive at the airport 30-minutes prior to departure and easily board and then not have to wait for bags on the other end, my day is a lot more pleasant.

1 item that fits in the overhead, and one item (the personal) which fits under the seat in front of you. Are there any other airlines that, for coach anyways, who allows more?

I see that a lot. Or more specifically, a maximum-size roller and a slightly-smaller roller. Ain't no way that second roller is going to fit under a seat!

I always figured it had to do with 1) fees and 2) if you let everyone board with all their stuff, space will run out sooner than it does now, slowing down the whole process even more.

I agree.
I have found that Americans carry on much more hand baggage than people in most other countries. I hate taking flights in the US because of that.

In response to the OP - Mostly, it's the airlines that set their respective baggage allowances, not the countries.