Google is no saint, but there's absolutely no reason to ascribe ill intent to collecting phone numbers of 2FA setup. The reason is simple: Google has billions of users, and at any given time, a lot of them break their devices and lose access to 2FA credentials. Phone numbers, despite all their flaws, are still the most reliable long-term and mostly-immutable attributes which can service as a proxy for identity which can and does aid account recovery at scale. If you crack your phone screen, you can walk to a brick and mortar cell shop, present your ID and get a new phone that receives security codes without a second thought. If you're using Aegis and storing MFA seeds locally, you're on the hook for backups and no one wants that responsibility.
Think of it like using social security numbers to authenticate yourself to the bank. Yes, it's terrible, but it's kind of the only thing that works when done on a massive scale. Yes, you can do better at managing your 2FA credentials, but most users cannot - they struggle even having strong passwords. Phone numbers bridge that security-usability gap. To be clear, this isn't an endorsement of the system (I think the user should be allowed to choose), but rather trying to make sense from an engineering perspective.