Differential signal isolation
I would not say that "usually" there is galvanic isolation of the RS-485 bus. Only in severe applications that can bear the cost or where the lines are very long and common mode voltages are difficult to control. 

As far as whether isolation of just the power supply is equivalent- you've mentioned the main reason why it isn't - usually there is something else connected to the circuit on top of the power supply and RS-485 connections. Even if there isn't, noise coupled in on the RS-485 lines would drive the circuit relative to the (presumably grounded) housing/chassis so you could get issues from capacitive coupling of that noise to anywhere in your circuit. Of course you could add a shield, connected to the RS-485 ground so the entire circuit was encased in a Faraday cage at the common-mode voltage of the communication bus but there is still coupling through the power supply input-output capacitance, which can often be in the nF or tens of nF range for switching circuits (more like pF for typical RS-485 isolators, so 1,000-10,000x less).  

Which is not to say that there are not possible situations where it might be appropriate, but it would be a fraction of a fraction of RS-485 applications.