Because there's more to a generator then just charging batteries. For example my watermaker is 240v, and while it's generally ok to run while underway, it's not recommended.
That said, there is an unwritten rule -and some cases a written rule, that should be respected regarding generator use.

I only ever run mine when underway and already motoring. I have found a great solution to evening 240v requirements ( now cook onboard with an induction cooktop along with other 240v uses) in the Ecoflow delta max portable lithium battery unit which is both a 168a lithium battery, 2400w sine wave inverter, solar panel regulator and super fast computer controller charger. It means in periods when my solar panels are not keeping up that running my generator for around an 1 hour usually whilst already motoring ( so very little noise difference) can put about 130amps into the unit. I would never run a generator that disturbs others and completely sympathise with the writer.