GM exists in a very competitive industry with major competitors at or above their capacity, while Google has maybe 1 or 2 competitors in several of various industries, and dominates several independently and almost completely.
GM might be able to claw aware market-share from competitors, and end up more valuable, but not by that much. Google is in the sort of position that allows them to potentially capture a brand new industry if one appears.
GM could (relatively) easily get broken up or get massively devalued to to a large market event or by falling behind in an industry that is rapidly changing shape from what they are accustomed to. Google falling out of existence/dominance would almost have to be the result of a fundamental societal shift at this point (or anti-monopoly legislation somehow), and they could throw enormous amounts of cash at problems for years before facing insolvency (from my understanding).