It doesn't, people have just decided RDM should never melee for more reasons than the DOT they don't do, and as such they are making more and more barriers that other jobs don't have to justify how they're still right.
I meant think about it:
PDT build - can cap at 50% - Phalanx adds to that, stoneskin improves it further.
MDT built - can cap at 50% - Magic Defence Bonus improves that, Bar spells improve it further