Actually, it is.
To consider the Earth as round, you have to make multiple assumptions. The most important one is scientifically consistency is important. Sure, you may say that one is insane if he rejects consistency, but that is also a judgment. If you did not value consistency as important, you could come up with something other than "The Earth is round."
Whether we view something as a 'fact' is subjective. We put value on each argument tossed in our direction.
While some positions can only be rejected in a way that society will view you as insane or mentally ill, but that's also a judgment. No one is objectively sane or insane. That's decided by the strongest. From en evolutionary standpoint, valuing consistency is superior and that is why it is important in our society. It could be otherwise.
Hence why fact are subjective. I may view as a proof something that is not to you.