Why do employers prefer employees with good work ethics?

As someone who is apart of a family business, we look for employees with good work ethics because we want people to do their best at their job.
When they do their best, employers know that the job is getting done correctly, and efficiently. They don't want to pay someone who slacks off. 

A good worth ethic is something like 
-Showing up to work on time/Actually showing up
-Staying busy
-Doing their best, even when people aren't looking
-Honesty

Etc...

Hope this helps!