If sections are all expanded, one has to scroll an unknown distance to get to the section one is interested in. If sections were to expanded by default, it would probably be better to not make them collapsable at all and instead include in the mobile layout the table of contents found in the desktop layout. However the table of contents uses anchor links which may be less usable in a mobile context (easier to lose your sense of place, a user's desire to not click links that may load a new page for performance and/or bandwidth reasons).
I agree that an "expand all" option would be a useful addition, I didn't follow the link in another post that presumably explains why one isn't being added.
User: This doesn't work on your mobile site, why don't you just give me your desktop site because it works better?
Mobile web site dev: We can't do that on mobile because X.
User: But X is already a solved problem on the existing site.
Mobile web site dev: Sorry, but we need to do a mobile site.
Collapsibility means requiring high-precision clicking, instead of low-precision sweeping of the finger.
I see your point in general, but even the average user browses Wikipedia often enough to realize they're always in-page links. If only by accident, I hit the wrong link often enough :)
Why aren't the sections already opened by default? Why aren't there any images? My phone out-performs desktops from just a few years back. I don't need this super-minimalist wikipedia. I just need something that's slightly more responsive because I don't have the horizontal space to handle all of its columns properly.