>>38331717It started because it was originally marketed to stop sagging breasts or to help prevent it. Eventually there was a mammary fixation which started making training bras a thing for young girls to get used to and as "fashionable/coming of age", rather than the undershirts they typically wore until they could fit an adult bra.
Its a mixture of blood circulation due to training bra/sports bras for development of the breasts and just because its a societal norm anymore.