Why is ordinary soap and water adequate to decontaminate possible nerve agents?
Nerve agents are usually susceptible to hydrolysis leading to non-toxic degradation products

Most modern nerve agents are based on fluorophosphonate cores. That is they contain a $\ce{PFO(OR)(X)}$ core where R is some organic radical (-iPr in sarin) and X can be a range of radicals (Me- in sarin, OR or N=R in novichok agents).

Most are not very stable in storage as they are subject to degradation from hydrolysis so have to made not long before use or have to be made ultra pure to avoid any contaminants catalysing their degradation by acid or base hydrolysis. As a result any were deployed as binary agents (where two tables components are mixed inside the weapon to create the active ingredient).

The key to their reactivity is the P-F bond which is easily hydrolysed to give much less harmful products. This hydrolysis is not fast enough to degrade the compounds inside the body before they kill you but is fast enough that they don't hang about for long periods in the environment after use (it is worth bearing in mind that they are extremely powerful inhibitors of acetylcholinesterase, without which your nervous system can't function: it doesn't take a big dose or a long time for them to work). It is also something that can be accelerated by washing. It seems likely, for example, that mildly contaminated clothing can be decontaminated by a standard domestic clothes washing cycle as this would dramatically accelerate any hydrolysis reactions. 

It is also worth noting that, if the contamination were large enough, you would notice it very rapidly. So any residual contamination in people not already ill would be of a very low level and easily rendered insignificant by washing.-