I used to use Stack Exchange a lot with an email-based login, but I haven't been active for a couple years. When I came here just now I had an Inbox with messages, and the trophy icon showed me a list of my achievements. So the site obviously knows who I am. But when I tried to join this community, it asked me to create a new account using my Google id. Why couldn't I join as-is? Is there a way to do that, or associate this new account with my old one?
But when I tried to join this community, it asked me to create a new account using my Google id.
Besides that you seem to use your old account and are logged in to post this question, I am not a 100% sure what you are talking about.
This is what I see when joining a new community:
I am logged in with Google automatically with my browser (Chrome).
Were you asked to create a Google ID additionally?
Is there a way to do that, or associate this new account with my old one?
As the picture above clearly states
We will automatically link this account with your accounts on other Stack Exchange sites.
And this is what apparently has happened already. If I check your profile here, I see this:
Going to your Stack Overflow account profile, I see this: