Companies, especially traded ones, have to provide constant growth to make shareholders happy.
Constant growth is not really sustainable: when you sell one device to each possible user, you saturate the market.
So Apple has to release version-specific features (hardware and software) to nudge people towards new hardware.
Personally I am not aware of any hardware limitation that would make Shared Photo Library “impossible” on a Mac running Monterey, but there are probably privacy issues.
I just updated my Mac to Ventura today, but I won’t be using the Shared Photo Library because I don’t like the idea that my photos are kept in someone else’s iCloud account, even that if a family member (and vice versa).
I will keep using shared albums.
If you really want the feature, your options at the moment are getting a Ventura-compatible mac or using a legacy patcher to install Ventura on your iMac.