[icelancer]

Right, the liability is not "we are going to send a letter that says you're too old/you're a woman" or something blatant like that, the liability is that a very well-meaning person sends a thoughtful rejection letter and it can be inferred in the language - true or not, and this is the kicker - that there is discrimination going on.

You are doing a nice thing by being detailed, but you are basically introducing nearly unbounded downside for the upside of being nice. Most companies don't find much value in this calculus. It doesn't make it the right thing to do, but it is understandable.

[Apocryphon]

I doubt that liability is much more of an excuse- how often do disgruntled candidates actually sue corporations, in any context? If a company doesn't want a candidate, there's not much value lost by them burning bridges with them, by refusing to send them feedback, or even a rejection notice. This is especially true for big companies who receive a lot of candidates. And for smaller startups, they simply lack the time and energy to give detailed responses to people they pass on. A failed candidate is of marginal use to a company.