Duckduckgo could easily be fully owned and operated by some three letter agency. The NSA is already able to go onsite and tap into the data that passes through corporations and they've been doing exactly that for decades (see Room 641A) and they can force corporations to keep silent about it using national security letters. You should already assume that every US based company is sending every scrap of data you give them to the state.
With no way to avoid your data from going to the state, what are you left with? Worries over companies collecting, selling, and using your data against you. That's a very real and perfectly valid concern.
We know that other search engines are doing those things, so it's best not to use them if we can avoid it. Duckduckgo might be doing those things, which at least gives us a chance, and even if they are it'd be better to hand your data over to several different companies than to give them all to one source (like Google for example) because the more data points any one company has on you the more control they have over you.
The worst case scenario would be that Duckduckgo is actually secretly run by Google and the data being collecting from the service is being used to help fill your dossier at Google but if that's the case we're never going to know about it until a whistleblower comes forward.
As defeatist as this all sounds, I do believe in taking steps to try to protect your privacy where you can, and I take many steps that go far beyond what most people are willing to, but we also have to accept the reality of the situation we have where our laws and regulations do not protect us, and there is very little we can do to protect ourselves but depend on others to do what they say. That's why I use duckduckgo right now. not because it's trustworthy (we can't know that), but because they might be and that's (sadly) the best option we have at the moment.