Why aren't Alkali metals be used as combustion fuels with water? (such as Lithium or Rubidium)?

The energy required to isolate pure alkali metals (by electrolysis) is greater than the energy produced by their subsequent rxn. It is only in specialized applications that group 1 metals are used as an energy source (lithium batteries).

They are relatively rare and the results of the reaction are a solid (or solution in water) which is extremely dangerous.

When lithium is used in batteries, it's energy storage ability can be re-used many hundreds of times, with relative safety.

With the new lithium battery technology now being tested, a battery can be recharged hundreds of thousands of times without degradation.