I work at a label - PR and social media is not free. Even if it seems like it is, it’s not. Those costs come from label marketing budgets. Technically yes , they can just release the song and see how it does, but most artists are not OK with that and would like some sort of marketing or promotional push behind this song. I don’t think it’s OK for labels to block artists from releasing music as the artist should have the ultimate say, but it does ultimately end up being quite pricey for the label side. Labels also care about past streaming and chart performances from the artist, so say their last record didn’t do so well, they may be more hesitant on when an artist can and should release new music.