On the contrary, users get very angry if stuff isn't fresh.
Someone changes trump article to say he is a poopy head. If that gets fixed in 2 seconds, no big deal. If that gets cached, and the edit to fix it doesnt hit the caches for a couple hours, wikipedia is now the top story on CNN.
Generally wikipedia caches are expected to be updated within seconds or minutes at most.
I think my larger points still stand. In comparison, almost all other services at the scale of wikipedia have critical almost-realtime components, and is almost useless without the possibility to authenticate users (which can't really be cached).
Not saying that the people who manage to keep Wikipedia so stable are doing an easy task, just that it's very different from almost all other things on the web.
Assuming things havent changed, each varnish server listens for purges via multicast udp.
You can see per-server stats on purges happening here: