The main reason is that Google finds synonyms and paraphrasing. The XenForo search function does not.
Please add support for synonyms to XenForo.
Elastic Search allows webmaster to define specific synonyms so the functionality is there. The issue is that it does not have a list of millions of synonym definitions included and there is no way for a webmaster to add this.
I know its probably a complex matter to add this, but IMHO it's a crucial matter.