Distilling all the explanations, the answer is simple.
When you're limited to one case, all-uppercase has a long history and we're used to reading everything from headlines and titles to telegrams and stone inscriptions in uppercase. It's natural, we're used to it, and uppercase came first historically anyways.
Whereas all-lowercase isn't really a thing historically. You see some trendy logos or ads that use all-lowercase, but that's a pretty new thing. (Well, and then maybe back to e.e. Cummings for poetic effect?)
At the end of the day, it's frequent to encounter text in all-caps, it's rare to encounter it in all-lowercase. So that's why.
Historically the distinction between upper and lower case has been one between different scripts for carved/engraved letters (where curved lines are awkward), and handwriting which does not have any such limitations.
BUT THEN IF ALL-UPPERCASE WERE THE HISTORICAL DEFAULT, WHY DID MOST OF THE WESTERN WORLD SWITCH TO USING NEARLY ALL LOWERCASE? AND WHY DOES IT FEEL SO JARRING - EVEN RUDE - TO USE ALL CAPS?
> WHY DID MOST OF THE WESTERN WORLD SWITCH TO USING NEARLY ALL LOWERCASE
It's easier to handwrite than with all Latin capitals. Also, the greater variation in size of characters makes it easier to read at higher text densities, like on the printed or written page.
And from there, it spread mostly via imitation of style. If you look at most of the scripts in the world that don't have upper/lowercase distinction, they tend to be far less blocky than uppercase Latin script, and more like lowercase.
> AND WHY DOES IT FEEL SO JARRING - EVEN RUDE - TO USE ALL CAPS?
Because we are trained to interpret all-uppercase today as form of emphasis. Emphasizing everything, especially when it's not necessary, indicates a lack of social awareness. It would be similarly socially awkward to end every sentence in an exclamation mark.
Because the distance between text and reader shortened after the arrival of the printing press.
In Roman and pre Gutenberg eras, text was inscribed, eg on monuments. The reader might be a casual gawker, or someone at the base of a tower, or generally someone who could not be assumed to be in a position to engage closely with the text.
Once print became widespread, that changed and the superior readability of mixed case won out- it was faster and easier to read. All caps was reserved for things that had to be consumable at speed, at distance and in short chunks, like newspaper headlines and mass media posters. Over time the use of upper case became synonymous with IMPORTANT and since shouting people inherently think their message is important, they used upper case too.
COBOL code is an interesting case, actually. One key reason why early computer languages were either verbose or cryptic is that they had very few standard symbols and punctuation to play with. It's why COBOL still has syntax like DIVIDE X INTO Y GIVING Z - you might not have a standard forward slash or divide symbol in your character set. Also why PASCAL (drawing from early ALGOL) went with BEGIN and END markers as opposed to the later use of braces.
I'm pretty sure lowercase letters originated with the creation of the Carolingian miniscule script[1].
I had heard/read that it was created as a unification of handwriting under the Carolingian empire, but that may be untrue[2]:
“The use of the new script makes an experimental impression,” says the Latin scholar. “They were trying it out. In the Middle Ages a script like this was not just invented, as is the case today. It was developed as part of the living tradition of a scriptorium. In the 8th century Corbie was something akin to a laboratory for new scripts.” This is another point that militates against the idea propounded by many popular history books that in the framework of his cultural and educational policies Charlemagne more or less commissioned the devising of the minuscule with a view to creating a uniform, readily legible script.
Prior to that, people wrote in uncial and half-uncial. The distinction between majuscule and miniscule letters wasn't there yet, but already then, they were enlarging and decorating letters at the beginning of the sentence back then. See this image on Wikipedia for an example[3]: https://en.wikipedia.org/wiki/Uncial_script#/media/File%3AKe...
I like to also imagine that since telegrams were supposed to be short and direct, it was the written equivalent of yelling over a bullhorn. Anything more eloquent would be written by hand in proper casing (and cursive)
It's only jarring if the rest of the context is not all caps. Precisely zero people in the 80s complained that their VCR is shouting at them because its display is all caps letters.
Also in older computers, fonts were designed for all caps text. And it was fine:
Historically yes - uppercases came first. Even when lower cases showed up, the combination key sequence to get the lowercases working was harder so that took more time to catchup
When you're limited to one case, all-uppercase has a long history and we're used to reading everything from headlines and titles to telegrams and stone inscriptions in uppercase. It's natural, we're used to it, and uppercase came first historically anyways.
Whereas all-lowercase isn't really a thing historically. You see some trendy logos or ads that use all-lowercase, but that's a pretty new thing. (Well, and then maybe back to e.e. Cummings for poetic effect?)
At the end of the day, it's frequent to encounter text in all-caps, it's rare to encounter it in all-lowercase. So that's why.