Comment by gambiting
3 years ago
Because while the courts agree that legally licences are transferable, they haven't ruled that Valve and others should build a technical facility to allow this. Microsoft put themselves in this position by accident because they allow licence keys to be activated multiple times - so you're free to sell your licence to someone else and Microsoft has to activate it. While Valve can just refuse to build this functionality into Steam and no court has ordered them to do so.
This is incidentally often the real reason why big companies refuse to release seemingly 'obvious' features their customers would value.
Because of some adversarial legal precedent in a major market that would impair them more then the expected benefit.
Microsoft probably is moving away from selling permanent licenses partly due to this, for everything except those to large customers they can individually keep tabs on.