Why can't water and oil blend together?

Because water and oil have different molecular structures.

A water molecule consists of an oxygen atom and two hydrogen atoms, both of which are connected to the oxygen atom, and there are two pairs of electrons in the outermost layer of the oxygen atom that are not paired. Water molecules are strongly polar molecules. The main component of oil is higher fatty acid glyceride, which is very weak in polarity and can almost be regarded as nonpolar. According to the principle of similar miscibility, oil and water are immiscible.

Whether these two liquids are mutually soluble is closely related to their molecular polarity. There are usually two kinds of interaction forces between liquid molecules: van der Waals force and hydrogen bond, which make molecules attract each other and keep a certain distance. They are determined by the chemical and electrical properties of molecules.