There are plenty of really bad hires who cost a company lost time and money because they simply are not competent. It's made even worse because many incompetent people are genuinely nice and pleasant people, they are open minded and may even sincerely care to learn, and it's very hard and demoralizing to fire them and can take a big emotional toll on many people.
Firing an asshole who is incompetent is one of the easiest things to do and it can be done well within the probationary period, heck even an asshole who is fairly competent isn't a big deal and can be a complete morale booster and likely will not have resulted in much lost productivity.
I'd say most of the people I fire are not so called "HR liabilities", and in general most people are good people. But most software developers are not competent and in some cases that realization can take a year or longer in domains that require a lot of ramp up time, training, and investment. It's often a lot better to avoid hiring people who have even a 20% chance of not working out.