They don't want to do this, because they don't want to become a "stamp of approval" for companies coming out of YC.
Specifically, if a company comes out of YC and is looking to raise further funding down the road, it becomes an important data point if YC decided to continue investing with them or not. By not participating in future rounds as a policy, they avoid this potential issue.
Specifically, if a company comes out of YC and is looking to raise further funding down the road, it becomes an important data point if YC decided to continue investing with them or not. By not participating in future rounds as a policy, they avoid this potential issue.