1) not all girls want that kind of attention so they choose a more modest bathing suit.
2) even the girls that do want that attention, usually have such a low self esteem from media, magazines, rediculous messaging to be ultra skinny, that they don't think they look good...even if they are gorgeous. and girls that crave that kind of attention anyway usaully have some other issues going on in their head like low self esteem.
3) some beaches are more family oriented and it wouldn't be decent.
but me personally, I don't like thongs as a bikini or underwear. I like ordinary style. it is sexier ot me and leaves a little bit to the imagination. if your gonna be a in thong and tiny fabric triangles covering your nipples, you might as well be naked. I would rather slowly peel off the regular kind than someone hand me a wrapped present and tell me what it is before I can unwrap it. lol.