If you're working on the content and the looks at the same time, and you don't 100% know where the project is going, it's helpful to consider them as a combined unit, so you can alter and/or discard that unit without delving into separate CSS and HTML trees. (Or, worse, not being sure what HTML or CSS you should alter or throw away when its corresponding CSS or HTML ends up in the garbage).
Separate content and presentation is a great final outcome, but when a project is in flux as it's being built, it's a burden.
EDIT: Moreover, it's not just that the "ship has sailed long ago"; the ship was attacked and sunk. Frameworks that combine CSS and HTML aren't doing so out of neglect; they're aggressively rejecting the separate looks/content paradigm.
Separate content and presentation is a great final outcome, but when a project is in flux as it's being built, it's a burden.
EDIT: Moreover, it's not just that the "ship has sailed long ago"; the ship was attacked and sunk. Frameworks that combine CSS and HTML aren't doing so out of neglect; they're aggressively rejecting the separate looks/content paradigm.