You can never measure response rate directly. There will always be people who see your ad and then respond later (possibly by some other ad on a different platform that they wouldn't have responded to in the first place otherwise). This might or might not matter to you.
The large companies who buy ads have statistical departments tracking their ad response. They know how well the ads work to close enough for their purposes. Those departments don't need the direct response rate because there are better measures.
You can never measure response rate directly. There will always be people who see your ad and then respond later (possibly by some other ad on a different platform that they wouldn't have responded to in the first place otherwise). This might or might not matter to you.
The large companies who buy ads have statistical departments tracking their ad response. They know how well the ads work to close enough for their purposes. Those departments don't need the direct response rate because there are better measures.