It took me quite a while to realize caps were considered shouting. (I may be a little slow)
How did this come about? Did someone sit down one-day ans say: Let's proclaim from now on that caps are rude and shouting?
How else is one supposed to interpret all caps? Trying reading something written in all caps and see if it's possible for you to interpret it as anything other than yelling. Let me give you an example, a simple sentence.
Don't you get it? I made a joke, it's supposed to be humorous.
That doesn't seem that bad does it? Now let me write the same statement in all caps
DON'T YOU GET IT? I MADE A JOKE, IT'S SUPPOSED TO BE HUMOROUS.
Both sentences are the same, but the first one seems polite, the second one seems antagonistic and even insulting.
The only difference between the two is that the second statement is in all caps.
There's also the fact that writing in all caps can be extremely difficult to read, so writing a long paragraph in all caps is rather rude because people are going to have a hard time reading it.
It's also easier to read text which is all lower case than it is to read text which is all upper case, so if you're going to avoid switching back and forth from upper case to lower case, maybe because one is a really bad typist, then it is definitely much better to write everything in all lower case than to write it in all caps.