With elements with a large number of style rules, you add a lot of very long lines to your html.
This would be a nightmare to work on/maintain.
In the HTML, sure. But so what? It gets compressed in transport so there won't be significant overhead there (similar to how remote desktop protocols that send images with compression turn out to be just as efficient as those that tried to send intelligent drawing commands), and the browser has to compute the final style for each element anyway so there's no overhead in-memory either.
> This would be a nightmare to work on/maintain.
HTML is an output format, you don't edit/maintain HTML any more than you edit/maintain binary machine code. And keeping the styles inline allows better encapsulation: the styling for each component lives in that component, you don't have the spooky action at a distance of child selectors.