I think you overestimate the time and effort involved.

a 1% chance to earn 20k more a year and learn new skills along the way is worth more than the $200/a few hours of my life a strict cost/benefit would indicate.

I think you may be right for jobs you _are_ underqualified for, but what about all the jobs you only _think_ you're underqualified for?

You really have no way of knowing if you're underqualified or not from the amount of information about any particular job you might come across (on a job board or a company's website). Now obviously they may list a bunch of technologies you've never worked for, but even then they might be willing to let a smart person learn.

Agreed. You may also be ruling yourself out of consideration for future opportunities with the same company that you genuinely are qualified for, because you've created the perception that you're incompetent or just don't interview well. (Let's face it, it would be difficult to give good answers at the interview when you're clearly not qualified.)