You can tell from auto-complete many people add 'wiki' or 'wikipedia' to searches at Google. And with the growth of Google-search-from-browser fields, I would also strongly expect that most people who type reformulated queries during an extended Wikipedia session do so through Google, not Wikipedia's much weaker onsite search.
Perhaps is related to the kind of subjects you are searching for; or it may just personalization by google.
Or maybe we are both biased for ultimately silly reasons and the truth lies somewhere in the middle but the lack of available data reduces everything to mere speculation.
This 'little test' on 3 arbitrary early-20th-century historical names, only looking at the top 4 suggestions, doesn't 'show otherwise'. I said 'many people', not 'most'.
Turn off 'instant' so you see the top 10 autocompletions, and watch over all your queries. You will very often see "wiki" as a suggested suffix.
Or better yet, just add " w" to the end of any of your own tests: " wiki" will be the top suggestion, which demonstrates that 'many' people add it as a suffix on Google.
Everything on this question is not reduced to bias and 'mere speculation'. I've observed many peoples' search behavior, not just my own, and habitual recourse to browser-based search boxes or always-requery-at-Google is growing over time (especially with the rise of Chrome and its 'onebox').
Wikipedia also did usability studies in the 2009-2010 timeframe, from which Wikimedia director/developer Erik Moeller reported: "our test subjects tended to resort to common web search engines to navigate Wikipedia instead of using the site’s own search" [1]. (Wikipedia has since moved the site search box to help it be found, and I suspect that's boosted its use, but it's still subtle compared to the always-available, always-familiar in-browser Google-powered search.)
