It’s because slavery is the most evil institution ever created by humanity, not a fun way to buy girlfriends who won’t say no, hope this helps
looking at you, Japan
Japan seems to at least partially include it as the horrors of the past, as it often reflects how things worked in Japan's history with added magical enforcement, and more often then no the MC is anti-slavery.
It is not like they are one of the nations that still practice it in one form or another, there are plenty of places people can buy wives, or people in general, to this day.
It's been a constant for years. Especially with fantasy and isekai. The first I remember in modern anime was Shield Hero then it just was everywhere. It's tiring.
It was there before, but Shield Hero is the one that popularized it.
Most MCs that partake do not use it well, Shield Hero who was only taking paret out of desperation, and planned to leave them MUCH better off then he found them.