Why don't more designers make flyers with a 1:1 dimensions / Facebook covers?

Just wondering why graphic designers do not create flyers with 1:1 dimensions and facebook cover dimensions.

I feel like a majority of people buying the flyers are going to use it on Facebook / Instagram … and 6.25 x 4.25 are not suitable for these platforms … I tend to just buy flyers that come with a cover photo aswell so the event page does not end up looking awkward and unprofessional.