Why is it so expensive for companies to replace workers?

In modern times it is possible that an employee could have a number of different jobs in a short amount of time. Do you think this frequent job changing could skew results for this type of ideal employee selection? Do you think potential candidates can use these screening mechanisms to their advantage by making themselves seem like perfect candidates when in fact they are not?

What personality traits may not seem like a good fit based on an initial screening but in fact would make a good employee?

Do you feel that hard work and dedication could overcome a person-job mismatch?

Please use references as per APA 6 style