Except I'd be fired if I decided I didn't feel like working 40 hours a week every single week without taking paid time off. Hours are tracked for full time salaried employees, of course, that's been the case for literally every single job I worked. (And I'm no spring chicken)
If it worked the way you claim, then what would prevent me from working 2 hours a week and be like "lol, I'm salaried, pay me a full time salary for those two hours?"
https://www.shrm.org/resourcesandtools/tools-and-samples/hr-...
>The Department of Labor (DOL) states in the 2004 overtime exemptions preamble to the final regulations that employers may require exempt employees to record and track hours and to work a specified schedule: “We agree that employers, without affecting their employees’ exempt status, may take deductions from accrued leave accounts; may require exempt employees to record and track hours; may require exempt employees to work a specified schedule; and may implement across-the-board changes in schedule under certain circumstances.”
"Exempt" employment status largely works against the employee in practice.