My personal interpretation
This is not always true. I don't know how to determine content and what specific categories are being compared to but I personally feel Diigo + Google's sidebar, SocialMentions' Matrix, Mahalo's Article meets Twitter, Diigo's drop down show several contents of highlights and Google with Twitter Search Results on Google greasemonkey script are some of the few things that beat out default Google.
Problem is, Google is very built in. It almost feels like a native app.
To paraphrase an ICarly joke:
What's the default search engine on your browser? Google.com
What does Google Reader remind you of when you check your RSS feeds? Google.com
What does Google Docs remind you of when you browse your documents? Google.com
What does Gmail remind you of when you browse your e-mail? Google.com
What is one of the fastest alternative at getting to a Wikipedia article if you're too lazy to type the full site? Google.com
What do most privacy conscious alternative search engines steal their results from the most? Google.com
Obviously power users can fix this but Google is just shy of Twitter as far as avoiding it's influence.
All other search engines either try to be aggregators or search engines, if you use at least one other non-Google search engine service, it's almost impossible to not just settle for Google on quick searches. It doesn't hurt that the term is ingrained as a verb.
The more you avoid Google, the more you'll just get annoyed at people telling you to Google it. Even if you avoid everything Google, topics like this still make you want to click on Google just to "verify" the claim.
It doesn't hurt that eventually you'll still find quick search results to be much more comfortable on Google and in-depth results too as they have the biggest content and the most active ranking changes.
For the most part, other search engines are only best if it's middle ground filtering. I love the way Clusty's filtering works and Yahoo has a better way of searching for CC'd images.