They were copying Odo to make him comfortable.
At all times? They looked the way they did all the time unless they specifically looked like someone else.
Why would they care about making Odo comfortable when he wasn't there?
Also Blob pointed out that those who recognized Odo as a shapeshifter shouldn't have.
Yeah, the female changeling used that form around the Dominion all the time regardless of whether Odo was there, which leads me to believe she/they'd never really assumed their own humanoid form until Odo came around, and just settled on that for familiarity with their allies, such as the Cardassians and Vorta.
Which makes even less sense that they were always recognized as Founders/changelings. I recall it was mentioned that the Founders were rarely seen and almost a myth, and they despised solids, so it's doubtful they spent much time in humanoid form at all.
And yet Odo was recognized as a shapeshifter based purely on his appearance at several times when it made absolutely no sense. Perhaps not all the time, but there were definitely times when he was recognized when he definitely shouldn't have been.