Why is it that a full bridge rectifier requires input to be derived from a transformer that provides dc isolation from the supply?
A full wave bridge rectifier requires input to be derived from a transformer that provides dc isolation from the supply in situations where it is desirable to:


connect the negative point of the supply to earth.
protect the supply and connected circuitry from earth faults.
have no point in the supply and connected circuitry to have a voltage with respect to earth that is higher than the supply voltage.


Some or all of the above may not be necessary and there are other means for satisfying the requirements such as providing the isolation at a latter stage such as demonstrated by the answer provided by @Oldfart. Therefore the requirement is not a firm requirement that applies in all circumstances.