
I think they mentioned because they don't want their coworkers seeing/treating them differently. I don't think they're worried their coworkers would judge them, it's more this this their professional environment and the both of them thrive on their departments just the way they are now. They probably wouldn't want the constant teasing and jokes and they probably think they couldn't keep up the "professional" relationship they greet each other with in the office.

I feel like once they get to a serious point in the relationship like about to marry or move in they should just tell everyone I mean they're gonna invite those people to the wedding most likely. but it will probably be revealed beforehand because some rival comes in and so one of the two of them is gonna just tell everyone they're dating so everyone knows they're taken lol
can someone remind me why are they keeping it a secret again? I don't think their co-workers would mind. I guess they're gonna wait till they're super serious like moving in or engagement or marriage?