Because we consider a slit which is much narrower than the wavelength of the waves passing through it, physically there is no way for the waves to distinguish the slit from a tiny point source of waves. This is known as Huygen's principle. Mathematically, it is the same principle which makes the electric potential at a distance r far away from a distribution of charges which are spread out over a small distance d<<r go just like the field from a single point charge, k Q/r, regardless of exactly how the charges are arranged.