Company pays you money. You work for them. They ask you to do something you're good at. Sounds good to me.
Result: all rational incentive is to do a terrible job on those deliverables that are least pleasant and/or least rewarded.
It's a simple point. I'm surprised you missed it.
There're many other tasks like this - usually the reward for being good at your work is to get more work. And the way out isn't to be shitty about doing your work, it's to ask for what you want in return for doing well at your work. That could be a raise, or the freedom to work on interesting projects of your own choosing, or more personal time, or it could just be "I don't want to do interviews this quarter so I can focus on my professional goals".
I think there's a legit problem at Google in that interviewing isn't rewarded highly come performance review time, so it doesn't translate into bigger raises or better projects. But there're mechanisms in place for saying "Please don't send me interviews" or blocking off days where you won't be scheduled for one. If the best interviewers take advantage of them and then HR finds out that there's a shortage of good interviewers, that sends a strong signal to the company that they need to increase the rewards for interviewing.
It is the sort of ROI that can scale a company from 20 to 20000 employees in just a couple of years :)
If you're spending a lot of employee hours filtering candidates who could have been filtered more easily through other means, I can understand feeling a bit indifferent about it.
(or whatever the actual percentage was, I don't know the total)
That said it sounds like the quality of candidates sent to her were really low and the upstream recruiters should have been working to improver that.