TenForward: Where Every Vulcan Knows Your Name

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.

Iirc the rest of his group were called the grand communion of something, and they had trained and practiced shapeshifting for many years through their lives (centuries iirc) so they find it incredibly easy to adopt any form they choose, and they choose humanoid when interacting with lesser advanced humanoid forms.

Poor Odo is essentially a lost child that was raised by wolves (the DS9 community) and as such he's a noob shapeshifter.

*bajorans, but yes

They originally appeared like that so as to be immediately identifiable as Odos people, then just stuck with the forms solids had seen them in so as to be recognized- whenever they didn’t want to be recognized, they weren’t.

Maybe he was born with it...

I wanna say that's their generalized shape from when they were solids and it's their default form. Odo, the female changeling and Las all have that similar facial structure

My head-canon is that the Founders met the Progenitors somewhere along the way and hard-coded a kind of “default” solid look into their collective matrix that resembles the Progenitors. The Founders emulate them in a lot of ways (affecting the evolutionary trajectory of life forms like the Jem’hadar and Vorta) and so “honor” their work by trying to look like them when they take on a humanoid form. Given this, Odo looks the way he does because that’s what a solid form Changeling is intended to look like.

(This is my nerdy way of reconciling that the Progenitor in TNG is played by the same actress as the Female Changeling in DS9 and both have very similar make up effects)