Why do companies ask about my motivations?

The last thing I want is to recruit somebody who ultimately gets into the organisation and finds it's not a fit. Maybe they stay, but their morale is low and they're not happy. I'm not able to look after them very well. And they don't produce very well for the organisation. I don't really want to take that risk. Whereas here, yes, it means I need to work harder because I need to convince the candidate that I'm worthy of their attention, but that works for me because it means I've given them a story that they can then choose to accept, in which case there's a good chance they're a good fit, or decline in which case it's as well they walk away.