These websites are credible because, I can trust on the content and the authors who have contributed their research papers. They are qualified research scholars and above all, they provide the most valuable information about the scientific research that is going on all across the world.
Well, in my opinion, credibility, which can literally be defined as believability is more important when you look for the right information about a particular disease, research or online products, etc. However, when you browse sites just for fun, you do not bother about what content or design a website is having. For example, if I search about diabetes, I would definitely look for an official site where I could get the right information. However, if I search for quotations, jokes, or just for fun, I would least bother about the