Your point is valid, in that every other motor vehicle that operates on public property has a requirement to be able to be positively identified to a owning entity. But there is no requirement that this be identifiable from great distance.
A drone in the air does create a unique challenge that a transponder would help resolve. But just like any other form of identification, it's an 'honor system' in that I could just put any vessel ID number on my boat, could steal a tag, or transmit a fake transponder ID if I were engaging in illegal activity. A trivial matter if I am advanced enough to be conducting operations with unmanned aircraft.
So I'm not sure that resolving someone's curiosity about my legal activity is a compelling enough reason to require me to attach a broadcasting identification device to my drone, or otherwise identify myself or my aircraft to law enforcement 'just because they should know'
Transponder codes aren't just going to be used by lawful citizens, but specifically lawful citizens that are careful and read the instructions (and have bought a drone with proper instructions or they independently looked them up). With how few people read instructions, I don't think this is a good solution.
The N number is there for when the drone crashes into something/someone. They had dropped the requirement for a while - looks like it is back on again.