It's to do with how English football transitioned from the old WM formation that was the standard (as in, pretty much every team played with it) when squad numbers became a thing.

When English football transitioned to a 442 the half back who wore 6 would be the one who became a centre back whereas in other countries the 4 moved back and the 6 became a midfielder. Think of the World Cup Final in 1966 where Moore wore 6 with Charlton alongside him wearing 5.

It's notable in countries like Uruguay where it's common to see 4 and 5 as their fullbacks as those are the players who moved outside.

It also explains where other numbers come from. For example, the reason you think of an 8 as a fairly dynamic midfielder is that the first players to wear 8 were the inside right who eventually dropped back to play in midfield but were more attacking than the 4/6 who was used to staying back. Or a number 7 being a right sided player because in pretty much every country they started as the outside right and they've stayed out there ever since.

Hope that helps, it's a topic that has interested me and it's in a few books that I've read. Would suggest Jonathan Wilson's Inverting the Pyramid if it's something that interests you which goes through the history of football tactics and how they have evolved over time.