Because then you have 200 blog posts and suddenly you realize "oh I wanna add an author section to all my blog posts". That is 200 files to edit. Could you write a script to do the edit? Sure, but then you pretty much build a static site generator.
`sed -i` is one command: https://www.gnu.org/software/sed/manual/sed.html#index-_002d...
`sed -i` is one command: https://www.gnu.org/software/sed/manual/sed.html#index-_002d...