No, it is not.
First, you must understand that a website can be a blog, that a blog is a website but a website is not necessarily a blog :-).
Where does this myth come from, then ?
Open source solutions such as Dotclear or WordPress make possible to put websites (blogs) online very easily. These very well designed solutions are ‘Search Engine Friendly’.
Indeed, they do not present any constraint to obtain a good SEO. This is a great leap forward compared to the many ‘personal websites’ that are often full of errors.
Finally, the very principle of a blog promotes good visibility : content, frequent updates, many links ‘exchanged’ between bloggers, etc.