This is bcoz they are 100 % ionised and the dissocation constant approaches to infinity.
For eg, take the case of NaOH.
Since NaOH is a strong electrolyte, it will produce Na+ and OH-. Now initailly, when Na+ reacted with H2O, it had also produce H+ (besides NaOH) and this H+ will reacts with OH- and again give H2O. Thus finally in the solution Na+ and H2O would be contained just like in the original mixture.