In reply to Norris.

50% of the sites you mention are hosted on WordPress.com, which has a paid option for custom css.

JetPack, which is active on over 1 million sites, has a custom css option.

A lot of themes also contain an option for adding custom css. These range from the "mega" themes on Themeforest to more simple themes like those by ArrayThemes.

The reason for 'bloating' core is to make things easier for users; core committers and the lead developer for this release have outlined why they want it added on trac, Slack, and this blog post.