Why do girls were training bras then

I would say is because moms are worried about pervs who enjoy staring at young girls developing breasts. I know people put camisoles on their five year olds because you can see thru their tops.

I personally find that a tight undersized tank top will help with a modest male breast development. Worn under a regular T-shirt ect.

In the case of our Daugher, it was to "train" her to get use to the etiquette of being a young lady and to keep pervs view proof.