It is obviously simply that way to dumb-down the show for the sake of not confusing the audience. Same reason why the female Founder, Laas, and other Founders always look like Odo even though they hate humanoids and
are good shapeshifters, hence that makes no sense whatosever on many levels.
Also, there are more sensible practical implications. It's not like they are gonna give RA's lines to the other actors. That would have been a tragic waste of RA's amazing talents.