Yes, in the original agreement wages are absolutely the consideration. The point is here is that — all too often IME — companies will, shortly after you start, ask for another signature on an additional agreement, giving them (the company) additional things. The sexual harassment pamphlet example is mild (and more about me not signing something falsely rather than a bad deal), but I've also seen IP agreements and stock agreements after the fact.
If you consider only what has changed from the original agreement: the company gets whatever it is you're signing the document for. You typically get nothing, aside from getting to keep your job. It's in this second agreement that I just don't find a compelling argument that "keep your job" (i.e., wages) suffices as "consideration". If you do, it's effectively reneging on the original deal, and trying to replace it with a new one that's not as good for the employee. Meanwhile, the employee has likely left their previous employment, and is at a huge disadvantage to walk away at this point. Particularly so when a prospective employee asks for anything that requires a signature up front, and when the documents in question use legalese like "this is the entire agreement". After that, there shouldn't be any more agreements unless something actually changes.
If you consider only what has changed from the original agreement: the company gets whatever it is you're signing the document for. You typically get nothing, aside from getting to keep your job. It's in this second agreement that I just don't find a compelling argument that "keep your job" (i.e., wages) suffices as "consideration". If you do, it's effectively reneging on the original deal, and trying to replace it with a new one that's not as good for the employee. Meanwhile, the employee has likely left their previous employment, and is at a huge disadvantage to walk away at this point. Particularly so when a prospective employee asks for anything that requires a signature up front, and when the documents in question use legalese like "this is the entire agreement". After that, there shouldn't be any more agreements unless something actually changes.