Here’s what I found:
It takes very little energy to remove that outermost electron from an alkali metal. Thus, alkali metals easily lose their outermost electron to become a +1 ion. ... In fact, as you go down the 1A column, the first ionization energies get lower and lower, making cesium the most easily ionized element on the periodic table.
So basically it’s because part of what makes alkali metals so reactive is that they have one electron in their outermost electron layer.