Because son, when operating commercial operations you may at times be required to give a commentary (or emergency brief) to your passengers.
You do not want them having the ability to easily communicate with you at times when you're trying to fly the aeroplane (or talk with each other for that matter via intercom distracting you).
Yes if you were just taking a friend for a jolly and they were in the right seat you would want them to have a mic.
This question and application will not be for that purpose. Someone wanting these types of headsets will be utilizing it for the purposes I mentioned above. The company or operator will be paying.
Thinking clearly now can you imagine flying 7 excited passengers in a single operating scenic flights with boom mics on their headsets plugged into the intercom system and operating into a busy CTAF?
Hence my sentiments
Fortunately on more modern aeroplanes you can isolate the intercom.