The US soccer structure is very different from what you have in the UK.
Our largest youth program (particularly in western half of the country) is AYSO--with entirely volunteer coaches and referees. And AYSO has separate authority from USSF to train and certify referees. (There is a cross-over provision that allows a transfer from one to the other.)
Collecting volunteer refs is different from having people decide they want to do it for pay. Essentially, parents and older siblings get "volun-told" to be referees when teams are told they can't have uniforms until they get two ref volunteers. With that pool of volunteers, the three person system gets used even in 10U. While three refs are absolutely not needed in 10U, it is about as easy as an intro one can get to being a referee (well, except for the parents . . .). Being in the middle is even less scary with two ARs in support. And that makes doing the higher levels less scary, as it is a small jump up rather than starting at a higher level.
This creates a couple of dynamics. One is that parents paying a lot more to have their kid play club expect that if AYSO has three refs, they should have three refs. (Though I think that doesn't often happen at 10U.) But it also means that there are a lot of people with some volunteer ref experience--and many move on to ref for the club system or scholastic system. In other words, just as AYSO is the starting point for many soccer careers, it is the starting point for many soccer referee paths. (And unlike many rec leagues, AYSO does not permit the two-whistle system, so its refs are trained on proper three person mechanics.)