[Rudism]

I think the answer to your first point is easily explained by the fact that the major news orgs typically have paywalls that check the user's IP, cookies, and so forth to decide whether they're allowed to see the full article for free or if they have to pony up first. There's no way to really incorporate that functionality into RSS feeds so they're just pushing out the bare minimum as a Hail Mary to drive their dwindling RSS user base back to the website. Smaller personal blogs that don't rely on advertising or paid membership revenue to stay afloat are far more likely to provide full article content on their RSS feeds (since there's no reason not to).

Some feed readers have "scraping" support where they request each article's URL internally and figure out how (or can be configured with CSS selectors) to extract the article's text content and display it in the reader, though it can be pretty hit or miss whether that can work for a given site.

As for formatting, there are a couple possible explanations--the reader could be ignoring or mangling HTML tags in the content when it's displaying it, or the site itself may be generating its RSS feeds with mangled or missing formatting elements. In my experience both possibilities are equally likely.