The main reason for this is that companies do require having a registered address per office in most countries.
The site functionality is hard coded and mandatory as it's linked to many other functionalities in Bob (for example calendars / bank holidays / events), the Work table information (like employees working patterns and weekly hours for example), time zones, currencies, data in reports and so on 😁️
For instance, let's say you have one employee that works remotely in Portugal, for example. Their bank holiday calendar (and so their time off allowance) might be different from an employee that works remotely from the US. So on the homepage for example, under the "People's availability today" widget, you can see who's out due to bank holidays on different days.
You can always set up a general site and apply it to all employees if their bank holidays are all the same, as well as currency and time zones, but the best practice would definitely be to create different sites for the employees!
Common practice in these cases is naming the sites something like "Remote - Portugal". Many companies have fully remote work, but usually what happens is that they have their financial taxes based somewhere (and sometimes are even paid in different currencies, for example). So that could be the use case to choose their site. So you wouldn't exactly need to have an office in different places in order to have a site set up!
Addition: The main reason for this is that companies by law are required to have a registered address per office in most countries.