Because the network was explicitly designed to not allow this... otherwise it becomes subject to censorship, which is one of the main goals they try to prevent.
The (onion) address itself is never transmitted in plaintext through the Tor network... when you access an onion site, your Tor client encrypts the traffic multiple times, literally like an onion. No relay in the circuit knows the final destination.
Conversely, even if the official project implemented an onion blacklist, a fork would quickly appear to remove it. And node operators would likely prefer that one.
Anyone with any sense understands that introducing a node blacklist creates the capability to expand the use of that blacklist in the interest of political and/or military censorship. The Tor project, Tor devs, and node operators are adamantly opposed to any such censorship capabilities. Therefore it will not happen, period.
Edit: on second look I can see how you could think it was. I’m just proposing that if you run a node you be allowed to not become a rendezvous point for onion sites.