Employers don't require their employees to be members of a professional order because they think professional orders are nifty- It's because certain jobs are only legally allowed to be performed by a member of said order. If you were a dentist and ran your own clinic, you'd still need to be a member of a professional order (at least in Canada and the US afaik) to practice dentistry legally, which would come with obligations outside of your usual working hours.

Software engineering exists in a sort of gray area where you can often be a professional software engineer without having to be a member of any order, which is great in many ways. But I feel like one could argue that the informal expectation of software engineers to care about software outside of their work is similar to what is expected in other professions with more rigid governing bodies.