Say you write 10 blog articles. In HTML that's 10 files with some trivial cut & paste. In jekyll that's 10 markdown files and 1 HTML template. Equal time sink for both tasks.

Now you want to add Disqus comments to your blog. In HTML you edit 10 files. In jekyll that's probably 1 plugin. Same with adjusting theme, fonts, recent posts, generating RSS feed, recent Tweets, ads.