And trading via WhatsApp is also possible. And also illegal

The point of the regulation isn’t to cover every conceivable alternative. The point is to specify what avenue you can use to accomplish a task in a regulated manner

... and by the company enforcing it the way they enforce it, they can prove the employee bypassed the rules knowingly and purposely without the company being at fault.

> Why is trading via Zoom impossible, voice maybe, and text mandatory?

Because trading conversations have to be recorded (legally). It's easy for corporate policy to say "we don't record Zooms, (therefore) you cannot trade over Zoom". It's also easy to say "we record all text communications, (therefore) you can trade over text". Meanwhile, the phone rules predated any other modern form of communication so they have the complexity of "sometimes record and trade, sometimes don't" grandfathered in.