My experience with people who don't dot their i's and j's is from when they write in cursive
, when of course the pen generally stays down for the whole word, except to go back and dot i's and j's, cross t's and x's, or anything else I've forgotten to mention. I've seen people just skip all or part of that last step.
Why? I don't know. Maybe they're sloppy, in a rush, getting into the groove of letting their thoughts out, thinking it's perfectly legible without the extra effort, etc.