Except that the whole problem of WHOIS, that RDAP is solving, is that a WHOIS response an unstructured plaintext response, that's entirely arbitrarily formatted according to the whims of the TLD manager.
Ever tried to parse WHOIS data? You literally have to write a parser per TLD.
And things get even more stupid when you start talking about WHOIS records for IP ranges. Then you have to write a parser per IP-range delegation — starting at IANA, and working recursively, all the way down to the individual ASN. Where you have no idea how many delegating parties are going to be involved — and so get their own step in the chain, formatted however they wish — for any given IP address. (Ask me how I know.)
Ever tried to parse WHOIS data? You literally have to write a parser per TLD.
And things get even more stupid when you start talking about WHOIS records for IP ranges. Then you have to write a parser per IP-range delegation — starting at IANA, and working recursively, all the way down to the individual ASN. Where you have no idea how many delegating parties are going to be involved — and so get their own step in the chain, formatted however they wish — for any given IP address. (Ask me how I know.)