Siri isn't a search engine, it is a front end to a search engine[1]. That used to be Bing but now it is Google (see:
http://fortune.com/2017/09/25/google-bing-default-iphone/)
There was a time when the Siri folks approached Blekko (which was an actual search engine with its own index, crawler, and ranking Etc. to discuss partnering with Apple (personally I think they should have bought us :-)) But, according to people who should know, there was a cultural mental block at Apple about providing web services at the time. The biggest thing like that they had done was Apple Maps and it was a 'mixed' success. Apple didn't see itself as being a search company.
I used to point out that Microsoft had a phone (Nokia), an operating system (Windows Phone), and a search engine. Google had a phone (Nexus), an operating system (Android), and a search engine. Apple had a phone (iPhone) and an operating system (iOS).
Since that time Microsoft dropped the OS and phone, and Apple never did build a real search engine.
[1] More precisely it is a front end to a simple knowledge base, a local index of things on your device, and when those things are exhausted an internet search engine.