Because it’s easier to do a PR removing “slave” than to stop buying cheap things made in slave workshops.
It's "unpaid labour" now.
I genuinely don't see boycotts as powerful as others, so maybe I'm missing something here. However, shouldn't all avenues be taken if you're attempting to change something?
https://en.wikipedia.org/wiki/Boycott#Notable_boycotts
You're right that boycotts without leverage experience little success, but wrong if you think they are never successful.