I am not so sure the scale is main reason. Most wordpress sites are static content and run db+site on same server. They are scaled using caching which sort of m

I am not so sure the scale is main reason. Most wordpress sites are static content and run db+site on same server. They are scaled using caching which sort of makes it like static website. The same approach is used with big flat file sites. Editing on dynamic portion and viewers see cashed site. Many big sites actualy use cmses as static site generators. But cashing is not that different.