Why does a carbocation undergo covalent bonding with, say, chloride, rather than ionic bonding, like what occurs between Na+ and Cl-?
Some carbocations do form true ionic compounds.  Many are known in which the positive charge is incorporated into an aromatic ring (e.g., cyclopropenyl, tropyl) or conjugated with multiple aromatic rings (e.g., triphenylmethyl).  But in most carbocatioms you have a compact, relatively low-energy vacant orbital that will overlap to form a bond with an electron pair from, say, a chloride ion.  Alkali metals don't have such compact valence orbitals and thus do not form a strong covalent bond even with the positive charge all on one atom.