Isn't it because an under 5 has to be with their parent, they can't even walk a little way away from them so it would be impossible to meet with another adult for exercise. Also a baby or toddler in a pushchair can't run to another person and accidentally get within 2m.
Surely the sensible suggestion for parents with older kids is to go on a walk with another adult with a similar aged kid and walk in pairs, kids together and adults together. You're only allowed to meet one other for exercise, not play, so would have to be walks really and not a play park, unless the adults are walking laps round the park for their exercise!