It all stems from a lack of yiras shamayim (clearly there is no need to have any yira from the social circle). A woman who has yiras shamayim wont dress contrary to halacha and a man with yiras shamayim wont tolerate it as a husband/father.
=======================================================
“These girls are taught to be tznius because they have to be, instead of wanting to be.”
It is irrelevant what they WANT. what is relevant is the ratzon hashem. Someone with yiras shamayim wants to do the ratzon hashem.
===========================================================
“do you think tznius improves with marriage, or did you just link two random issues?”
I dont think the 2 “issues” are as random as you make it seem. With an over emphasis on a girls physical appearance, to the point where a girls photo on her shidduch profile is standard operating procedure, is there any wonder that girls continue to push the envelope in the appearance department (this includes clothes and makeup). the lack of yiras shamayim in our bachurim who are seem to have no issue with this runaway train moving forward at breakneck speed.
============================================================
“Popa, is it better than ten years ago, or is it just better than fifty years ago?”
It isnt better, or worse. It is different. Both are just as bad. Sure, we took steps forward, but we also took two steps back.