Of course such things exist, and you don't even have to get that advanced when you can have one central CSS file which is linked in to every other page and use a variety of template pages that get pasted in. The trouble is that, with each change you make, someone has to learn more of a markup language that, while pretty simple for those of us who use significantly more complex languages, is still precise and takes some persistence to use properly. Consider the difference if someone, let's call her Alice, has built a website for someone else, yes it's Bob. Now, Alice is busy doing something else, and Bob wants to add a new announcement to his website. How does he do that?
If Alice has written it in hand-coded HTML, the instructions probably look like this:
Write your new article in HTML. Save it as its own page, then modify the home page file to link to it with a summary. Make sure to link in the CSS file on your new page so they look the same. Then run this script which will replace the template lines with your standard navigation, header, and footer content. Once finished, upload all the new files to the server and test.
If Alice used Word Press, the instructions look like this:
Log in to the backend. Click new page, write the content in the boxes. If you need links or anything like that, there are buttons to add them right next to those boxes. Check the box to mention this on the front page, and if you want, adjust the summary. Click preview, and if it all looks good, click submit.
Which would Bob, who does not know HTML yet because Alice was doing that, want to do this time? For that matter, if Alice isn't the only typical maintainer, which one does she want to do if she doesn't want to see someone else mangle the scripts she's written for this? In most cases, Alice wants to have a CMS that has a GUI editor, perhaps because she's gotten tired of Bob calling her every time he wants a new page on the site because he can't be bothered to learn HTML and CSS to do it himself, but he can manage to type in some boxes. However, Alice doesn't necessarily have time to hand-code a CMS for Bob, so he ends up with one of the existing ones with a bit of customization on it.
I'd like for this to be different, but it's not. I know this from experience. I have written many backend systems in more complex languages to get around the basic options of HTML, but I do that for my own systems or internal things for my job. The thought of having to do that for other clients as a web designer sounds like a job I would dislike very much.