Because that's how it works. Formatting is applied to a full value, not one that hasn't been applied yet, since it changes the way the user's input appears. That would be very confusing if it happened while they were typing.
My previous reply here was not relevant to the question asked, which is: Why?
The answer is because these "text boxes" you're seeing don't really exist in the file. Acrobat interprets the text and tries to combine various parts of it to chunks that make sense, but this is not a 100% accurate process, and can never be. So if it sees two strings close to each other it "thinks" they probably belong together and "merges" them to one element. If they are further apart it might "think" they are not related and keep them as separate objects. This is a by-product of the fact that PDF files were never meant to be edited in this way in the first place.