Why can't I just book payroll as one number?

Because it isn't one number. A $50,000 payroll run might be $40,000 in gross wages, $3,000 in employer taxes, and $7,000 withheld from employees that you're holding for the IRS. That withholding is a liability, not an expense, until you remit it. Booking the bank total as a single wage expense overstates wages, hides your tax liability, and understates what your team actually costs.