Why are  RCS messages showing in the Queue?
RCS messages are showing in the queue because the RCS retry window is 30 days. This means that any messages not delivered due to certain reasons will continue to retry for up to 30 days.
What is the reason for the RCS failed messages?
Unreachable number" error in RCS. 
This issue can occur due to several reasons, including:
The recipient’s phone does not support RCS or has it disabled.
Network or internet connectivity problems.
RCS is not properly activated on your device.
The recipient may have changed their number or blocked messages.
A temporary issue with the Google Messages app.
The “Too many requests” status in RCS generally occurs when the same sender (brand/agent) sends too many RCS messages to the same recipient in a short duration. Google applies rate-limiting to avoid spam-like behavior. In short, the device/network flags the sender and temporarily blocks RCS messages to that number.