Is there a reason review solicitation has to be 1st party?

Is there a business reason or Google policy that requires review solicitation to only be 1st party? It seems like it would be beneficial to also solicit reviews to Google, Facebook, etc. in order to increase ratings and review counts with listings.

Hey Ben,

In order to comply with the recommended best practices of our partners in our Publisher Network, we do not support review solicitation on third-party sites. Please feel free to reach out to your Customer Success Manager for further information regarding this limitation.