Actually, to make it with cryptographic guarantees is pretty hard... I know of at least one university professor in the PET (Privacy Enhancing Technologies)/cryptography space who spent quite some time on his startup to develop such a search engine. In the end it all fell apart because of one the mathematical assumptions being unprovable.
This is just one example but I guess it illustrates pretty well why we've yet to see a cryptographically secure/private search engine as a product!