Payroll is a repetitive, data intensive, complicated and just not a lot of fun.
Owners and managers tend to delegate as much of the task as possible and let someone else take care of payroll.
Owners and managers tend to, if anything, review totals and check on payroll taxes. Some do not do that on a regular basis.
There are a number of ways people can steal during the payroll function. Some in-depth reviews, at least occasionally, may deter theft.