Because the first party vendor, in this case Tesla, doesn't provide that kind of mechanism?

That's what I'm saying though, instead of allowing permissioned access to their existing telemetry, they're publishing a platform that duplicates all of that functionality.

Tesla doesn't provide this. Also, its beside the point...

Which is: Tesla doesn't even want collect telemetry and then re-share it to 3rd parties. Tesla wants to provide a mechanism for the car to connect directly to a customer-authorised 3rd party telemetry collection service. This relieves Tesla from having to function as a middle-man and facilitate things like claims in tokens and granular permissions.