this also somewhat surprised me at first but I think it's obvious in hindsight, though they don't have to be a full-blown web browser (you can go read the epub specs at W3C to see what's supported)
as for (2) I'm not sure why you think it would make it less easier? being html, text reflows automatically based on screen size, font size, line height, etc
I guess I assumed that, for example, multi device support on websites for various device widths entails a bunch of CSS, which means the epub renderer would have to also do that, which basically means a whole web browser.
also that things like footnotes or anything that has a floating reference (table of contents links for example) might get very complex or require javascript
since ebooks are primarily (only?) text you don't have to worry about UI elements and such which simplifies a lot of the css
footnotes aren't really a thing with ebooks (at least as far as displaying the note on the page with the text). Because it is just a html renderer, footnotes are presented as mutual <a> elements located in the endnotes at the end of the book
as for (2) I'm not sure why you think it would make it less easier? being html, text reflows automatically based on screen size, font size, line height, etc