Why is it the valence electron that is removed from an atom upon ionization?
Because the valence electron is most removed from the nuclear charge.
Ionization energies increase across a Period (from left to right as the Table is faced), and decreases down a Group. Each neutral alkali metal has a single valence electron, and a reasonably well shielded nuclear charge. The low ionization energies reflect this electronic structure, but you will have to compare the ionization energies with alkaline earth metal and transition metals yourself.