Howdy Aleksey, I was wondering the same thing a while back, and collected and/or made a few block types, including Markdown, ReSTructured Text, and Code Syntax Highlighting via Pygments. You can see them here:
https://github.com/FlipperPA/wagtail-components
However, I think they're best kept separate because they'd introduce more library dependencies to Wagtail. When I asked myself if 90% of Wagtail's users would use these block types, my answer was no, and I think that's a good litmus test for whether or not to include something in core Wagtail.
Just my two cents,
Tim