Mostly comes down to population but the youth sports culture in the UK plays a big role too. There isn't a football/basketball and track connection in the UK like there is in the US - soccer is kind of an "all in" sport in the UK. If you read the bios of many of the top UK sprinters they all started out as soccer players but eventually switched to athletics after their speed on the pitch was noted by their coaches - they were never really 2 sport athletes like is common in the US.
Also, it's about to get a whole lot worse because of the UK selection committee's decisions over the past few years. Younger athletes are gonna either never go into athletics because it's seen as "not worth it" or just switch allegiance to another country.
They didn't start out as soccer players at all. They may have started out as players of Association Football and would eat Cornish Pasties and meat pies.