Because it doesn't suit them. It's like real estate: it's always either a buyer's market or a seller's market. Right now there are far more people who need jobs than there are jobs, and that really benefits those already established businesses. They can take their pick. They don't have to make themselves too attractive: if you don't want the job, then someone else does. The current labor market also castrates unions. Striking becomes a whole lot more effective if the strikers are confident that they can find employment elsewhere, and if there's not a bunch of willing workers ready to take the strikers' place. On the other hand -and this is a situation that benefits far more people- if there are more jobs available then laborers the circumstances flip. Suddenly jobs have to make themselves attractive to workers, rather than the other way around. Suddenly business owners' are more concerned about irritating their workers. Unless it's the business itself offering the job, new jobs just restrict a business's power.
Right now the power is in the hands of the smaller group, the owners. If there were more jobs than there are workers then the balance of power shifts. Workers begin to dictate the terms of their employment, rather then the other way around.
Broadly speaking, this is one of the many reasons why capitalism breeds socialism. It's simply a matter of which system most benefits the majority of the population.