I don't believe this would have been legal in many places. At least in the US, most cities have regulations on drivers and vehicles that are allowed to be used for ride-sharing and commercial purposes in general. For better or worse, the central arbiters that own ride-share apps verify that drivers and vehicles meet those requirements. A cab hailing protocol can't give you that, as you need some authority that has the ability to check whether you meet requirements before allowing you on the platform and it needs to have the ability to remove you as well. Being able to do that requires that there be a platform.