Odo was forced to take that form and it’s part of his trauma. While the other shapeshifters can change into humanoids without issue, they disdain the form and only choose to appear as one in support of Odo, who is a lost youngling.

All of their chosen human forms to represent themselves are variations of Odo’s. All other human forms used are perfect copies of their targets when needed to for infiltration and sabotage.