Because it's not actually unlimited in practice. After all, you would definitely be fired for taking an entire year off.
So, when a company says "unlimited," what it tends to mean in reality is "there's a limit, but we won't tell you what it is." This manifests itself in people with unlimited PTO taking (on average) less time off than people with defined number of days.
When you have a defined number of days, you know exactly what the limit is. In my experience, this results in people using most (if not all) of their vacation days.
I will note that this largely seems to be a US thing.
So, when a company says "unlimited," what it tends to mean in reality is "there's a limit, but we won't tell you what it is." This manifests itself in people with unlimited PTO taking (on average) less time off than people with defined number of days.
When you have a defined number of days, you know exactly what the limit is. In my experience, this results in people using most (if not all) of their vacation days.
I will note that this largely seems to be a US thing.