Why are artists uploading their full albums on YouTube, then how would their actual albums sell?
good point... they don't.
They get a small percentage of the advertising revenue (supposedly). A small percentage of something is better than all of nothing (albums don't really sell much at all these days). The real way for an artist to make money these days is through live shows but they have to attract an audience, which is another reason why they make their music available through social media.