Why ARP response is not broadcast?

An ARP broadcast is a broadcast packet, which means it goes all over the network. The ARP who-has IP_address tell source_IP_address packet asks everyone "who has this IP address". Only the holder of the IP address responds with the packet going to the IP address following the "tell". Therefore, the response only goes to the originating IP address, not the entire network.

What problem are you trying to solve and what the hell is a "big deal" in English or Metric terms?

I am just trying to understand the ARP protocol.

Broadcasting ARP request is easy to understand because the sender does not know who has the MAC-IP mapping. Receiver broadcasting ARP response is not a "big deal" which I mean that the ARP response does not look flooding a lot of traffic to the network. If ARP response is broadcasted, every host will have a chance to update their mapping table so they don't have to send a ARP request any more later on when they need it. But why in ARP protocol, the APR response is specifically sent back to the the requester only? What is the difference?

The ARP response is directed only to the node that made the ARP request in order to minimize the processing load on other nodes. Simple, eh?

Perhaps you should try thinking in terms of large networks where the majority of workstations do not communicate with each other but with servers. As the majority do not communicate with each other what is the point in spending cpu time in looking at arp responses and wasting network time, most arp caches default at 10 or 20 minutes so you would quickly lose the unwanted info anyway if you had a broadcast response, you would also have to ensure that the arp cache of each workstation was sufficiently large enough to hold the data for all the workstations in the network.