Think it's more of a North West thing in England, see a lot of kids wearing City shirts around here and United too tbf, not so much any of the London clubs. Real & Barca have the 2 biggest followings in the world so not surprising you see a lot of shirts, United's number 3, City 6th, Chelsea 7th, Liverpool 8th, Arsenal 10th
https://jobsinfootball.com/blog/most-supported-football-clubs-in-the-world/