If you have 100 candidates for a job, then doing a half-hour phone screen for all 100 of them is 50 hours of work -- a bit more than one whole work-week.

And if you're committed to not missing a candidate who might be really good but your heuristics would lead to a false negative on, how much does a half-hour phone screen really tell you? I mean, can you really reject more than half of the candidates based on a half-hour phone screen if you're committed to a very low false-negative rate?

What are you going to do, bring 50 candidates into the office for half-day interviews? You've now brought your sunk cost of interviewing from one person-week to 6 person-weeks. And you've also actually probably spent 3 months doing this process, so you've probably also delayed actually hiring someone for at least a month. 10 person-weeks worth of productivity starts looking a lot less good compared to a 10% productivity bonus.