Account level signals are nice but insufficient. There's a very long tail of low activity accounts that are difficult to distinguish from a bot. You can't really classify them as bots without generating a ton of false positives, and in general you'd rather let a possibly fake review through than piss off a genuine first time reviewer.
A google account cannot really be low activity if its owner browses the web, watches youtube, etc. The tiny minority who logs out of google when browsing, etc. could not leave a review, but it may be a small price for more valid reviews.
Google does have a big advantage in verifying good accounts through other activity, but that advantage shrinks dramatically for new accounts. There's still some things you can do, but at the end of the day you're only willing to assign so much risk to a brand new account based on metadata.