Another thing which leads to monopsony power is firm-specific training and skills. You learn bunch of stuff through on the job training which is valuable to your firm but which isn't really transferable to another firm. So you're more productive working for your company than you would be for another company, and they don't face competitive pressure to fully compensate you for your value.
As for why anyone clicks through a Megan McArdle post, I could not say.
