Being one word or one phrase long does not change anything. Here is a sentence with one word: “No.” Here is a sentence with one phrase: “Thank you.” Both require the first word to be capitalized. What’s relevant here is not the lack of a rule saying that one word or one phrase sentences must be capitalized but the lack of an exception saying that they need not be capitalized.
I’m all for the elasticity of rules. When the mobile version of the site started getting more use than the desktop version, I argued that we should be lenient on answers that started with a lowercase letter but were otherwise correctly punctuated and capitalized. This was because phones at the time wouldn’t automatically capitalize a word unless it directly followed a terminal punctuation mark (with no built in exception for the first word of a message).
I was overruled. Ben and Andrew wanted to keep the standards high, and it was decided that expecting people to manually capitalize the first letter of a message was no more demanding than expecting them to capitalize a proper noun in the middle of a sentence (which I think is fair, but I was more willing than others to make concessions to technology). These days, you actually have to manually remove capitalization from the first letter of a message on most phones, so the concession is no longer applicable.