Google's too disorganized and they don't have any supply chain tech built up. Amazon has years of supply chain infrastructure (both software, physical systems and organization) including supply/demand forecasting models and warehouse operational investment (Kiva systems, etc...).
The only differentiation that Google adds to Walmart is: - Top-talent engineering resources & brand which attracts talent to work on these problems - Layers of developer tooling and services that remove the "systems at scale" development curve of startups and smaller companies.
Now if Alibaba teamed up with Walmart... (read: acquired).