First, to answer the question why. The reason is actually pretty simple: selling 1 workstation license to individual clients does not cover our order processing and support costs. In other words, Veeam will lose money on these sales. All of our processes are designed around the fact that we're B2B company. While to make money off selling single desktop licenses requires B2C approaches such as selling online (instead of through resellers), having super cheap outsourced email-only support (instead of expensive in-house support with people on the phone 24/7), etc.
Now, specifically to your scenario, Matjaž. As a service provider, you are now able to rent licenses for all your clients. For example, if you have 10 clients with 1 desktop each, you can rent 10 workstations license from Veeam and deploy agents at your client environments using this license, which completely solves your issue.