It has to do with men hitting on them. They don't want to date and say it in the hopes that men won't catcall, won't flirt with them at work or the streets, don't ask them out, etc. If you think about it, it saves a lot of time. Or it would save a lot of time if the men around them listened to it and only hit on the women who want to date.

But now we are at a point that even saying "I have a boyfriend" is not enough for some men to give up.Maybe they think we are liying about the boyfriend and consider that single women doesn't have a right to reject them or maybe they just don't care since they only want sex and the women being not interested is not reason enough to say no. Who knows?

That kind of articles are written in that context.