They don't (https://www.twilio.com/en-us/messaging/channels/rcs, https://www.cm.com/rcs/, https://www.linkmobility.com/en-gb/channels/rcs, https://gatewayapi.com/rcs/, https://www.productsupport.esendex.com/what-is-rcs-messaging..., the list goes on).

Google makes RCS servers for carriers, host RCS themselves for carriers without RCS servers, and builds the most used RCS client app, so it makes sense that they're in the RCS monetization market as well, but you don't need to use them.

Google's pricing being displayed upfront while most others are listed as "contact us" will make Google much more attractive to small apps, of course.

Pretty much because the other major telcos tried to run their own rcs services and did so terribly