The problem is that at that point in the flow, it's owned by the SSO provider. The SSO provider can't know with certainty what account has an active account with the website.
In other words, they could definitely highlight or otherwise hint to you which of the Google accounts you've already approved/used via one or more of your authenticated Google accounts.
So, yes, they could do more to highlight _potential_ accounts but it's not the case that they have any visibility into the actual state of accounts.
I don't need to know for certain the account on the receiving side exists, just that I have signed in before with it. Facebook does this at least!
Like "has an account" isn't possible without leakage, but "has logged in through this flow before" (hell, stick timestamps in there too!) does.
One thought though: your SSO provider has that account list, but often prompts a re-login. So it could be that your SSO provider account picker _doesn't have access to your account information fully either_.