Because they are teenagers and they like to do things they know will give adults a sandy ****y. Many teenagers smoke and drink and screw around but all anyone seems to be worried about is teenagers vaping.
Seriously, on a planet that is supposed to be over 4 billion years old and has had teenagers since not long after the first humans appeared you would think that by now someone would have figured it out.
Stop drawing attention to it. No one ever complained about teenagers smoking in the days before vaping the way they do now about teenagers vaping.
Most of them are doing it for the attention. The more adults and the media rail against them the more likely they are to stick with it. Just be thankful they are not smoking or banging the cougar next door.