Because fairly frequently, I'll search for something on DDG, not find what I was expecting, and try on Google as a backup. And because in many people's minds, Google is search.
But honestly, if Google doesn't have it (down below whichever ad-driven first results I have to skip), it's a good bet that I'm not going to find it on other search engines either, unless some specific site indexes its own content better than the search engines do (archive.org comes to mind, for some things).
Of course, this is all colored by my search style and the topics I'm usually looking for information on.
DuckDuckGo is awesome.
!gi google images
!define the free dictionary
!r reddit
There are a lot more, and I find them quite useful.