Why are carbocations with sp3 hybridisation highly unstable? Why is it that sp3 hybridized carbonium ions do not exist? Or why are they very difficult to form?

For instance, nucleophilic substitution in bicyclo compounds at the bridge head carbon is not very easy. I don't understand the explanation given in the book. Thanks for any help.
When you hybridize towards $sp^3$, you use one $p$ orbital more compared to the $sp^2$ hybridization. 

The additional $p$ orbital raises the energy of the 4 $sp^3$ orbitals higher with respect to the energy level of $sp^2$-hybridized orbitals ($p$ orbitals are higher in energy than $s$ orbitals), which in the energy-minimizing world of chemistry is not favoured by the system. 

The energy is minimized by hybridizing in a $sp^2$ fashion (in which all the $\sigma$-bonding electrons go) and adopting a planar geometry.