More than likely because we never had the same problem with health care commuters from Spain.
I think you'll find that the reverse is the case now.
So English folk who retired to Spain for a few years and are now returning for whatever reason... could find they are out of the English NHS system for at least 18months and should possibly head for Scotland... if they have a significant health problem....looks to me like this dream of retirement to the sun stuff can get expensive.
If they take the trouble to comply with the EHIC system there should not be a problem.