I think it's because they're used to talking. There are absolutely pauses built into speech that are of indeterminate length or function, serving more as a space for social cues from your conversation partner. Depending on those cues, the space might be break for the other person, it might change what comes next, or it might simply be a pause. The elipses is a catch-all pause.

Now for written communication, none of that applies, but for short messages, people still expect to communicate casually. If you didn't grow up learning (and creating) ettiquette and convention for short message communication, I suppose you end up trying to emulate speech.