Of course, if your’re frequently creating page templates or website themes, you’re usually having a few reusable components prepared for yourself (I, too, have a very basic HTML template and a basic CSS file with the most general styles predefined (kinda like a simple version of what normalize does)). But, for example, these premade CSS frameworks make you change/write your HTML so that it matches the CSS selectors (through specific markup/nesting and non-semantic class attributes) which kinda turns around the actual intention of CSS. And likewise, these “Wordpress frameworks” just include almost everything so on short sight it saves you some development time but it bloats the sites and leads to overuse of irrelevant effects (fades, animations, etc.) just because it’s easy to implement them, and – much worse – leads to less thinking and intention of understanding on your own.
I think it’s better to have your own lightweight “framework” (like, basic templates, and/or reusable snippets) that specifically serve your needs, not require you to learn something new and complex and give up too much control in order to adapt to someone else’s style of coding.
Edit: Oh and by the way: I also think this whole “it comes SEO optimized” thing is overrated, too (as well as all the “SEO experts” that claim they will bring your site to the top in search engines, etc.). A well written and designed site with much relevant text content is search engine friendly by itself already. There are a few server/URL settings you can modify but that can also be part of your little personal snippet library. I can’t imagine what these frameworks do differently that makes “their SEO” more special.