My understanding is that there’s also considerably less admin for unlimited time off. In about 50% of states, your employer is required to compensate you for unused vacation days if you have a specific vacation policy where a # of days is attached to your position. If there are unlimited days I believe in many states employers can skirt this requirement.
In start-up culture where for some reason it’s standard to have no HR department, having unlimited vacation days means you don’t need to hire a designated person to process vacation day requests.
tldr; it’s a trap!!