It is up to whoever is paying for the service what they want and most don't want a public bus service. They want the flexibility to alter the times to suit their needs at short notice, they don't want the hassle of having to make arrangements to identify their staff in more formal measures for free or discounted travel to seperate them from the general public, they want to minimise the time their staff spend on the bus so they want the least stops and the most direct route between pick up points (which means they are even less useful as public services) and they want the vehicle they are paying for to have space for their own staff and not risk overloading issues if you open it up, or paying for a bigger bus than they need to ensure the space (if it is going anywhere it could provide a useful service to others then capacity becomes an issue otherwise there is no gain from making it public). Almost none of our works type services run to locations that would provide a useful or meaningful addition to the general network and most trips are not at times when anyone but the staff using it would want to travel.
The only reason to make them public is if you think there would be a significant additional passenger flow of the general public created by doing such to help defray the cost of the service, in most cases that is unlikely and for the businesses paying for these services the impact is insignificant to their site budget to provide a compromised resource, and you can't merge a flow into an existing service that doesn't exist (and in any case that would tie you to a single supplier who may not be willing to have the flexibility to meet your needs and in most likelihood will not be the cheapest operator so you may be paying more for something that doesn't provide exactly what you want).
As I have said previously there are certain things that would mean a service legally has to be registered where making it open to the public actually then brings an advantage is BSOG income, certain things that mean that they don't need to be, shouldn't or even if the most extreme examples can't be registered where it just brings cost and restrictions that may never be justified by any advantage from general public usage.