Several things. They work hard to get and keep their body's in shape and not so to they hide it. They enjoy knowing men are looking and admiring but they may be in a relationship or only interested if they think you really have something to offer. Higher end places, in affluent areas, you are likely to see a lot of trophy wives and gfs. In a manner of speaking their looks are their stock in trade and they feel they have to constantly keep at it to keep ahead of new and younger competition. I've seen women in this world in working out within a week or two of childbirth because they feel they Have to get back in shape ASAP.
They cannot afford even a hint of infidelity or interest elsewhere.
Working out in baggy clothes can create a hazzard in some cases. If they are serious about working out they don't want cloth flapping around. I wear the stretch exercisewear with a built in athletic bra to work in as well as work out because it is just more comfortable. I do get more male attention dress that way but that's not why I'm wearing it.
They may just wear it because they feel sexy in it just for themselves. It's like wearing sexy undies even though no one will see them.