- build a useable search engine
- show ads to users
User acquisition is based on word of mouth and a bit of guerrilla marketing: they are a search engine with decent quality that doesn't spy on you.
Not spying and not selling tracking data to others cost them some opportunities but gives them "free" users that would otherwise have stayed with Google.
The last few years Google has been busily lowering their quality so even if DDG haven't improved much they feel very close to Google these days. (Also, retrying in Google takes 2 seconds from DDG, while retrying in DDG after trying in Google first takes 15 seconds and more thinking.)