Because DNS works with many kinds of addresses, not just IPv4, and you shouldn't have to guess what type of address a record holds based on format.
When you see an MX record with the value 1::8, how should you interpret it? Is it a malformed value? Is it an IPv6? Is it some other protocol that you don't know about?
DNS clients shouldn't have to guess. Instead, the MX record should point to a domain name, and the client should look up the corresponding address record for the protocols it knows how to speak (A or AAAA or whatever else will exist).
That is the best most irrefutable answer for why it's impossible I've seen so far. Better than "because it's physically impossible". (Is it electrically prevented at the Physical Layer, otherwise the router would catch fire if you tried to do that?)
To be precise, it’s logically impossible, since MX records are defined by the relevant standards to consist of a number of bytes which are to be interpreted as a priority integer followed by a series of DNS domain name labels. There is no defined way for an MX record to contain an IP address. You could make up some such way, but then it wouldn’t be an MX record as commonly defined, it would be your private extension of the standards.
"The standard definition tells you not to" isn't as deep and complete and useful an explanation as "the standard definition tells you not to, because networking protocols should not have ambiguities or make guesses".
To be precise instead of tautological (which you should be when arguing the pedantic nuances of networking protocols), when you use a word like "physically", you're implying that it has something to do with the "Physical Layer", and that there could not exist another networking protocol in the same universe that DID allow IP addresses in MX records, because it's forbidden by the physical laws of nature, not by the logical laws of networking protocols.
It's literally like using the word "literally" to mean "figuratively". (And I mean "literally" literally, not figuratively.)
Obviously it's not physically impossible, because sometimes people do it, and sometimes it even works.
I will agree that it's unwise and scandalous and logically impossible, just not "physically" impossible, and that a better explanation is needed. That's probably why the first reply to your comment was "Out of curiosity though: why is it like this?"
A networking protocol that operates faster than the speed of light, or sends information backwards in time, could be physically impossible, according to the currently known laws of physics.
Although it would be super cool and convenient if you could specify a "Date:" field in the past or the future to send email at the specified time.
So was the term "Gish Gallop" that you wrongfully accused me of, so you still owe me a "mea culpa" for that too.
A series of statements needs to be lies in order to qualify for that term, and you totally refused to provide any evidence and failed to prove anything I said was not true, but it was. The fact that Eric S Raymond has provably and publicly said so many many many racists things over the more than three and a half decades that I've personally known him does not make me or WikiQuotes literally quoting a few of them a "Gish Gallop".
Those were all actual ESR quotes, despite your denials and overconfident claims that I was lying and that you could rationalize and explain away them all. Please, in the future, chose your words and heros more carefully, and stick to the facts, instead of making stuff up, and please stop lying and white-knighting in the defense of a virulent racist (and sexist Harvey Weinstein defender) and linking to his personal fundraising page.
Nobody "canceled" you -- quite the opposite: I tried to engage you in a conversation, gave you the facts and citations, asked you to explain yourself, and prove what you claimed, and YOU decided to refuse, despite the fact that you glibly brushed off all his racist statements and wrongly called me a liar by saying "I’m sure all your claims can be either explained as inconsequential or disproven as false" in his defense.
When you lie down with dogs, you get up with fleas.
When you see an MX record with the value 1::8, how should you interpret it? Is it a malformed value? Is it an IPv6? Is it some other protocol that you don't know about?
DNS clients shouldn't have to guess. Instead, the MX record should point to a domain name, and the client should look up the corresponding address record for the protocols it knows how to speak (A or AAAA or whatever else will exist).