Why is this fact alone not enough to avoid the accusation of plagiarism?

It's still a fact taken straight from a website and is not in your own words or at least cited

If you just used one fact from a website or even in your own words, it might cause plagiarism because it sounds exactly like something from a website

To avoid plagiarism with facts, putting it into your own words, or something that make sense to you, without using the same/similar words from the website.