That's the million dollar question. Tesla's highway performance has easily been good enough to do what MB is doing for several years. No one knows if it's a liability question, other legal/regulatory, Tesla (or Elon's) insistence that they get everything done first or something else.
I agree with you - level 3 on the highway, even in a limited capacity such as MB has would be a huge selling point and would likely drive a ton of subscriptions. For me, I enjoy using FSD around town, particularly with 12.3.6 since it's gotten so good for me but the monotonous highway driving is where I really appreciate it. Having the ability to read a book or do something else while driving on the interstate would be huge.