Why does the reactivity of the alkali metals decrease from cesium to lithium?

Alkali metals are more reactive since they are available in nature as combined state.

And from top to bottom reactivity increases due to increase in the size of metal and also forces of attraction between nucleus and valency electrons is more hence Valency electrons held less tigthly with nulceus.

Hence less energy (ionizationation) is need to removes electrons.

Hence lithium is least reactive and Cs is more reactive.