At one time we didn’t.
The earliest texts are written as a continuous stream of symbols. Not only weren’t there paragraphs, there weren’t periods, let alone commas, colons, semicolons, dashes, or any of the other punctuation marks we consider necessary today.
Yes, the ancients text messaged. LOL
Over time, writers adapted texts to make them more readable. It was a slow process, spanning centuries and cultures. Each change was an innovation that had to be invented, adopted, and standardized.
Paragraphing is an art rather than a science. At the same time, a book is a technology. Books have evolved with time just as punctuation did. Try reading a physical book from the 1890s. Most of them don’t feel right, though most people can’t put their finger on why. The height and width of a page, the aspects of a type font, the size of the font and the leading between lines, the brightness of the paper, the contrast of the ink on the page, every physical aspect that goes into the making of a book changes little by little with time. The choices we consider optimal today were not known or not possible then. Hence, old books read funny.
Paragraphs were adjusted for the experience of reading. They are heavily dependent on the width of a text line and the font used. A paragraph longer than a page loses all visual cues and is easy for the eye to get lost in. Very few writers refuse to break up text into smaller blocks, but there are always exceptions. The great writers can break any rules and experimental writers break rules just to force people to think about the act of reading. Everybody else is wrong.
Computer screens are much wider than book pages. Screen fonts are not optimal for reading. Contrast is much lower than book pages (unless screens are set too bright, in which case contrast is too great). That’s a major reason why book people complain about reading on screen. It is simply ergonomically worse.
You can get around some of these faults by changing font size or forcing text into columns. When you read the comments on articles or blogs, they are usually forced into narrow columns. This sends a cue to write brief text so that it doesn’t extend into an unreadable block.
Conversely, a site that sets the default width to the width of the computer screen, like the Dope, allows for long responses but most people would agree that posts should therefore be broken up into short paragraphs because they are more readable in this format. Not everybody does this, though. A few use no paragraphing at all, and they get abused for it.
Back to paragraphs being an art. A good paragraph tells a complete story. That’s why new paragraphs get the “topic sentences” that English teachers try to drum into you. The topic sentence breaks with the previous story and starts a new one. If you find yourself ending the story and starting a new one, that’s the place to start the new paragraph no matter how long the old one has been.
Or you can do so for effect.