There's cosmetic and there's cosmetic.
As PP stated, addressing a big hole in your face would be "cosmetic".
In the case of breast cancer, it's not about changing something that has always been to something they would prefer,
But returning something to the state it was before massive surgery.
That's not quite the same.
There is also the fact that rightly or wrongly, womens breasts are heavily sort of focussed on in our culture, they are seen as signifying a lot of things, and women report feeling really very much not themselves, not right, not whole when one or both are removed.
There's also being lopsided which must be a very unpleasant sesntation especially if you have larger breasts.
It would not really be publically acceptable to go out with one breast visibily missing > so there's time and ?money involved + the reminder of it all every time you get dressed.
Women's confidence, mental health can suffer also their relationships etc
In short it's not trivial at all and the oh it's only a tit it's pretty trivial type attitiude I think is maybe a bit misogynistic.
A child has bad squint, corrected on NHS, deemed cosmetic. But if it's not done, a massive effect on their whole life.
A person who has a realistic prosthetic instead of a very obvious one. Cosmetic.
A person who loses an ear in a fight. Cosmetic.
A person who has facial deformity treated. Cosmetic.
etc etc etc
I tihnk because it's breasts and women, it's easy for poeple to see it as vain, fundamentally because society is fucked up about womens breasts really.