Question

why are alkali metals highly reactive ?

Answer 1

they all have only one electron in their outer most shell,

Answer 2

Remember that accros a period, the atomic radii increases and so the effective nuclear charge ( attraction of positivity in the nucleus with the electrons). Hence for a group 1 element, it has the greatest atomic radius and the least effective nuclear charge and hence it is very easy for it to lose its electron(s) to make an ionic bond.