Because they don't know who is really is, and he needs to use a different voice around them too or, being smart, savvy men, they'll put two-and-two together eventually.
"Hey, you sound kinda like Bru...holy crap!"
I'm pretty sure I could recognize someone's mouth and chin pretty well, but you gotta suspend your disbelief on that aspect a bit, I guess. Hey, at least it's not hinging on a pair of glasses and a forehead spit curl. At least Batman makes a serious effort, and goes out of his way to properly separate his two identities.

t:
Perry White and Lois Lane. Seasoned journalists. Jimmy, with a photographer's eye.
None of them could put that together? Really?