I am not asking from the "Is Google evil?" perspective, I am only asking to find out why.
I have not found a significant reason to go back to Google. Search might be "hard" but my experience with DDG has told me that it is not so hard that it can't be solved by people other than Google.
Doing search well is extraordinarily hard, arbitrarily so depending on what approximation of "well" you're willing to accept. There are many factors which scale in computation very rapidly. The entire concept of meaning and relevance is also hard to reduce to a computational algorithm despite many valiant attempts to do so to a useful level.
That aside, if DDG doesn't work, changing search engines is unlikely to help much vs. trying a different query or consulting other humans. The world is not limited to Google. Google is just very large and entrenched to the point that it's hard to take on directly.