I don't think that what you're saying is true. It is my understanding that plaintiff lawyers often use the prospect of legal costs at trial to extort a large number of small settlements from large organizations. Arbitration costs are usually much less expensive than either side's legal fees, so covering those costs to avoid the prospect of a trial is advantageous to the employer (and the plaintiff).
None of that requires an arbitration agreement from the start. If it really benefits both you can just both agree to arbitration. Or make it opt-in at the start with an explanation of the great benefits.
It may benefit both parties, but it doesn't benefit either counsel. The plaintiff's counsel usually operates under an agreement which guarantees them a certain percentage of any resulting compensation (contingency), and financially punishes the plaintiff for dropping the matter. Once counsel is engaged on contingency, the matter only very rarely goes to arbitration (as arbitration eliminates the prospect of favorable settlement in advance of looming litigation).
