I've never come across a page on MDN I felt I could contribute to since it's so complete, but if I were to come across out of date docs I absolutely would. I imagine it's similar for a lot of folks.
Usually there are only a handful percents of overall users who actually create content compared to "read-only" users. There is nothing special about wikipedia, the same thing applies for sites like youtube,reddit, etc.
The same will apply for MDN as well, so if you haven't contributed there, this doesn't mean that nobody will.
This is the nature of any community platform [0]. Most people will just fix small issues they see or contribute a little in an area that appeals to them, and aren't interested in becoming heavily active. I don't see Wikipedia as special in this case, it's just so large that the 1% has enough critical mass to maintain it (well, mostly).
>Most people will just fix small issues they see or contribute a little in an area that appeals to them
Wikipedia makes this very difficult as well. I had to submit a small update to sales numbers for the best-selling video game franchises of all time THREE times because two separate admins reverted my edits for the most stupid reasons possible. Then they had some hand-wringing about the lack of sources (even though most of the data there wasn't sourced any better) before they stubbornly decided to stop reverting my edits. A less stubborn person would've given up editing Wikipedia after the first or even the second reversion.