Ye, like one VM running per city or region. I feel they are just sinking money into software and administration of software.
WhatsApp has shown that you can actually build large-scale system infrastructures very efficiently. And they had global service, Uber could profit from sharding as you suggest.
Apply for a job at Uber proposing a cut in engineering expenses with your plan and become instantly rich.
And never get hired because the people doing interviews have to protect their jobs :)