Yes, and FOSS payroll software also exists. People don't use it because they'd rather pay for a service that offers support (another service) and absolves them of liability (another service).
I'm not asking for a repeat, I'm asking you to substantiate your claim that "FOSS payroll software also exists". There is next to nothing available and certainly nothing that I would classify as "adequate".
This is just one example --- specialized business software is almost non-existent in open source. And I see no reason to believe this will change any time soon.
Most US businesses do their payroll in-house --- using proprietary software --- mainly due to cost reasons.
Companies that outsource payroll to a service provider are actually a minority --- mostly larger businesses.
https://www.civica.com/en-us/insights/managed-payroll-or-in-...