The core limitation is that the problem is dramatically more complex than it was when Google started. The number of sites were smaller, there was much less dynamic content, and there wasn't a sizable portion of the internet committed to an adversarial relationship with search engines forcing everyone else to go to the same extremes just to play catchup.

What this means is that you're looking for answers in a much larger search space, and the indicators you used to use are much less reliable. You have more resources to try to balance that out, but there's so much straight trash to weed through that it's pretty difficult to do.