This is an interesting question because under an apparently mundane topic it actually touches upon fundamental characteristics of flexible workforce countries such as for instance the US, Australia and to a lesser degree the UK.
The word "help" is actually a deliberate choice because it suggests that the helper will voluntarily step forward and is implicitly willingly bringing in some added value. A popular cliché for this kind of ad is that of a teenage helper working part-time in a Midwest hardware shop.
There is no commitment either on the duration of the job from the employer's side. It's just help; and it implicitly stops when it is not needed any more.
If you go back a few centuries ago when there were very few large companies and the vast majority of jobs on the market were proposed by farmers or small rural businesses, that makes perfect sense and can thus be interpreted in the context of the natural application of the law of demand and offer to the job marketplace.
Now that the contribution of medium to large size companies to employment is more significant, stable jobs are more dominant. As a result, the corresponding offers are thus more frequently formulated using the "hiring" word than the "help wanted" expression. Compare for instance the "position available" and "help wanted" phrases. The former does indeed look more stable than the latter.
A side conclusion is that, the "help wanted" sign being phrased by the job offerer reflects his own rationale and interest.
In contrast, it is also probably easy to explain why job requesters don't phrase their request in such terms as "good salary for life wanted". If that formula would work, you'd probably see more of these ;-)
Conversely you have very few of "best job in the world" offers. Although they seem to be quite successful ;-)