It's not entirely clear why they chose those forms (nor why they persisted in keeping them in later episodes) but the script direction for The Search, Part I gives us a solid clue. In short, they seem to have chosen to look like Odo because when Odo arrived at their homeworld, that's the form he'd chosen.
[Odo is walking toward the edge of what appears to be a large
GELATINOUS LAKE. The lake appears to be made of something other than
water. The surface undulates and roils in an unnatural fashion. Odo
stops at the edge. Kira stands next to him and there is a long moment
as Odo looks down at the strange liquid.]
[Suddenly a portion of the lake begins to MORPH. As Odo and Kira watch
in amazement, a group of FOUR SHAPE-SHIFTERS detach themselves from
the lake and take on the form of FOUR HUMANOIDS. Their faces have the
same "unfinished" look that Odo's does and they appear to wear simple
tunics. One FEMALE shapeshifter smiles at Odo and steps towards him.]
FEMALE: Welcome home.
At this point they're unaware of his incapacity to create a realistic face and presumably feel like this form is one that he's intentionally chosen and hence that he'll be most comfortable seeing, not realising that it's a sign of his immaturity as a shape-shifter.
In DS9: Chimera, we learn that an inability to cast faces is something of a trait among "the hundred" infant changelings that were sent out into the Galaxy.
ODO: A changeling.
6 THE CHANGELING (OPTICAL)
as it MORPHS into humanoid form. Although its face is similar to Odo's
in that its features are smooth and somewhat ill-defined, it's modeled
on a humanoid species we've never seen before.
It's definitely male, and we get the sense that he's a bit more adept
at mimicking faces than Odo, as his features are more expressive. He
wears alien-looking CLOTHES that are more practical than decorative.
They presumably also chose to portray themselves as male and female to reflect the fact that Kira and Odo are different genders.