Why is google not using a headless browser to craw

问题:

I'm aware of the steps it takes to make a client side website crawlable:

https://developers.google.com/webmasters/ajax-crawling/docs/getting-started?hl=nl

I just wonder, why isn't Google just integrating a headless browser in their crawlers to save us the pain of providing html snapshots via e.g. NodeJS and a server side DOM implementation?

I guess their might be a technical reason that I'm missing. I just don't see it.