It is, but the scientific method is the best we have to eliminate bias. If the study design is sound and the results reproduce it is a fact. Now it may well be the study design is biased, but since it is documented anyone who sees a new bias is welcome to design a new study that eliminates that (I write this as if doing a scientific study is easy, in the real world it isn't but I'll ignore that real objection). There are also interesting meta-studies that eliminate some bias.