Why won't Apple let me set duckduckgo as my default search engine for safari?

I am using a iPad Mini and Safari only lists three options for a default search engine (google, bing, yahoo), all of which lack privacy. I would like to set my default to duckduckgo and would prefer not to modify my iPad illegally or use secondary apps (duckduckgo app). Why won't Apple just let me modify my default search engine manually? I quite enjoy Safari and don't like wasting space with other browsers on my iPad.