Because it's always accompanied by human trafficking and rape. Even in countries where it's legal, the laws are shaped around an idea of allowing it, while preventing the worst aspects of it. The efficacy of various systems is still under review, as none of them are as effective as anyone would like.

It's a similar issue with polygamy. No one really cares what a group of consenting adults gets up to, but it always ends up devolving into some ones twelve year old daughter being married off to an old geezer, or a cult leader, or both. So it ends up getting banned.

They're both non problems that cause problems. And yes, a hundred ways to control the real problems have been tried. No one has really been happy with the results so far, there have just been various levels that local societies have decided to accept.