I wear baggy pants. I am not making a fashion statement (you'd know if you saw me); I'm wearing them because they are comfortable to me. And there are times where my wife would say that I need to hike my pants up because my drawers are showing.
Meanwhile, she'll wear yoga pants regularly. And she gets a lot more attention than I do - she was on a calendar set for a fitness club in her yoga bottoms being photographed and there was nothing but skin-tight yoga pant-wearing women all over and the cause is celebrated :humblebrag: - and it's far more revealing than my sagged pants.
So it can't be a 'decency' issue
I think it's just another law to satisfy voters/taxpayers with something that's more symbolic than meaningful. And people get to harass kids for another reason, to boot.