Man, people are so gullible.
A tiny patch of grass udpated 12 times per second... what happens when that patch turns into a field? How are you going to pass that kind of data over the internet fast enough? What happens when a million people are playing the game? Who's going to pay for all that CPU time? Micrsoft? They'd need to have dedicated processors ready to handle the workloads, the infrastructure required would be huge and costly.
What happens when the user is not online? The grass disappears?
Presumably further away grass will be on a lower quality local simulation.