You are being flippant here. Hiring someone into a tenure track position is a lot more work than hiring someone into a temporary position. Someone in a temporary position is usually just going to teach classes and that's it. All you need to know is if they are knowledgable and can run a classroom. Credentials give you a ballpark idea of the first, and references give you an idea of the second. Budget-wise you only need a commitment from above for one year's salary, which is a lot easier to secure than a continuing salary. You don't have to worry about their research compatibility, long-term career plans, or their ability to get tenure.
And if you screw up and hire a person who isn't up to the job, you don't renew them. So true, they are easier to fire, but that's a very small part of the story.