Because.. the Borg removed her body's waste extraction functions and her Borg armor did it for her? And the Doctor couldn't fix her so her so he devised a still suit/cat suit to process her waste in some medically nifty way?
Do you really think he wouldn't also have powerful smell molecule defeating technology integrated into this system? Because it would make little sense for the Doctor to be so invested in having 7 take socializing lessons if she smelled like poo. All the small talk in the world won't overcome that.
As a plus to your depressingly logical theory the cat suit detractors will have to finally shut up about it.