Let's also not forget that Scientists are also humans. Humans with their own beliefs and biases which do get transferred into studies. Peer review can help reduce that but since peers are also humans with their own biases, but also common biases shared amongst humans it's not bulletproof either.

There will always be some level of bias which clouds judgement, or makes you see/think things that aren't objectively true, sometimes it comes with good intention, others not so much. It's always there though, and probably always will be. The key to good science is making it as minimal as possible.