That virtually all science is socially constructed is an important truth, but from that it is does not follow that all social constructions are scientific. But I've never heard any social theorist assert that they are.
I'll take your word for it. I haven't read any sociology of science beyond Max Weber's Wissenschaft als Beruf
first-hand, and am going by a general impression I'm getting from newspaper articles on the subject. My impression is that modern sociologists seem to think that scientific findings are a lot more arbitrary than they really are. I am also getting a sense that a lot of this alleged arbitrariness comes from a sentiment that, oh well, it's all socially constructed anyway.
Maybe it is. But suppose, for example, that an Indian tribe tries to veto the examination of a 5000-year-old skeleton on the grounds that it's one of their ancestors, and DNA analysis shows that it's not. Does that mean you retreat to relativism? ("They've got their socially-constructed truth and we've got ours") Or does the DNA verdict win out because mismatching genomes are hard facts whereas the revelations of some Indian elders are not? To me it's clearly the former. But I'm not sure at all that all sociologists would agree.