Google voice is great, why is it free?

I've used Google Voice at least 10 years, probably spent hundreds of hours in calls and many tens or hundreds of hours in Hangouts for voice and screen sharing (mainly work, though Hangouts went away) before Teams or Zoom became popular.

I don't get how they could offer Google Voice for so long for free. I put $10 in my account back when I signed up for my number way back then and not a cent has ever been charged by Google, which is amazing but odd.