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.
I thought the same thing, then I realized there was a significant selection bias: I was only looking at events where half the hypothesis (DDG fails & Google succeeds) is true. For example, how does DDG perform when Google fails?
That's true, but I wasn't thinking in that direction. I was just trying to answer "why are people still using Google?".
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.
If Google fails doubt it will be found on other choices. I will post something on a smaller site and Google will pick it up while the other search engines will not. So for me just easy choice is to use Google.
Suppose I might as well configure my browser to use that pattern, instead of going directly to Google search, too. That would make it a bit more convenient to remember.