I suspect (it was introduced before I started, and re-introduced after I left) that it's there to remind users of the work that software performs on their behalf, much like how TurboTax says "$X in taxes saved" or Brave says "9 hours saved blocking ads" or your brokerage says "Total profit: $X". Users tend to forget this, and just assume that they're entitled to the world where the product exists.

If the number was in the thousands in the late-90s, you could probably get through all of the sites.

If the caching was put in later, say, the early 2000s when google started getting really big, the number might just keep growing until it eventually looks ridiculous.

I've never worked at google, but I've seen a few systems that broke in weirder ways when nobody bothered to maintain them after they had outlived their usefulness.