If the lawyers didn’t have this definition in their head there would be no drive to make the software agent a purchaser, because it’s a stupid idea.
I enjoy reading both sides of the argument when the arguments make sense. This is something else.
Lawyers don't come up with good ideas; their role is to explain why your good ideas are illegal. There's a good argument that AI agents cannot exercise legal agency. At the end of the day, corporations and partnerships are just piles of "natural persons" (you know, the type that mostly has two hands, two feet, a head, etc.).
The fact that corporate persons can have agency relationships does not necessarily mean that hypothetical computer persons can have agency relationships for this reason.
Indeed, agency (the capability to act) and autonomy (the freedom to choose those actions) are separate things.
BTW, attorneys' autonomy varies, depending on the circumstances and what you hired them to do. For example, they can be trustees of a trust you establish.