Why are carbocations with sp3 hybridisation highly unstable?
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.