Well they kind've do. Tweet-chains. If someone replies to their own tweet to create a series of Tweets, Twitter groups them into a chain and puts a "Show This Thread" link to expand it. It's the same thing in practice.
It is not the same thing in practice. It doesn't allow as natural paragraph formatting or sentence flow since the break-up interval for the text is dictated.
It makes it harder for people to reply to the chain (do you reply to the first or last?). The same problem exists for retweets; if someone retweets a tweet partway through the thread, it doesn't behave as neatly as you describe iirc.
Also, there's just a difference for the author. Writing a paragraph of text like this comment, where I get to go back through it and edit it as I go, add and remove paragraphs... It results in me creating a more carefully edited and thoughtful piece of writing.
With the tweet threads, someone either has to write the tweets one at a time (and can't edit previous ones) with twitter dictating the breaks, or they have to do paragraph-oriented editing locally, and then figure out how to break it up correctly for tweets (which is non-trivial actually! URLs take up a difficult-to-predict-naively number of characters, so it's not naively 140 chars if you have urls, etc).
Basically, twitter's model facilitates a different sort of writing and reading than a single long post does, even if they are just both a bunch of letters.
It makes it harder for people to reply to the chain (do you reply to the first or last?). The same problem exists for retweets; if someone retweets a tweet partway through the thread, it doesn't behave as neatly as you describe iirc.
Also, there's just a difference for the author. Writing a paragraph of text like this comment, where I get to go back through it and edit it as I go, add and remove paragraphs... It results in me creating a more carefully edited and thoughtful piece of writing. With the tweet threads, someone either has to write the tweets one at a time (and can't edit previous ones) with twitter dictating the breaks, or they have to do paragraph-oriented editing locally, and then figure out how to break it up correctly for tweets (which is non-trivial actually! URLs take up a difficult-to-predict-naively number of characters, so it's not naively 140 chars if you have urls, etc).
Basically, twitter's model facilitates a different sort of writing and reading than a single long post does, even if they are just both a bunch of letters.