Why are old websites so unsettling?

Obviously I didn't feel that way at the time when I was growing up in the 2000s, but now when I visit older websites I get this strange feeling like things aren't quite right. I don't know what it is, like I'll never know what I'm about to see on the next page, the colors that were used, the fonts, Etc. it just feels off. Maybe a web design or just design person in general can explain the psychology behind it? Because I don't know how to explain what the feeling is.