An alternative explanation: even if they did spend more time on the resumes they wouldn't be selecting better, because they don't know what to look for

Ah, the sheer ego size of hiring managers who go around saying stuff like "understanding how candidates think" or "assessing cultural fit"

The brightest minds of psychology - with far, far more resources and time - can't replicate those results if they run the tests this Monday or this Wednesday... and then comes the hiring manager and they say they can do it with a CV, some technical assignment, and an hour of chat. After having spent less time properly studying the topic than highschoolers spend on their homework each week