People often start multi-tasking because they believe it will help them get more done. Those gains never materialize; instead, efficiency is degraded. However, it provides emotional gratification as a side-effect. (Multi-tasking moves the pleasure of procrastination inside the period of work.) This side-effect is enough to keep people committed to multi-tasking despite worsening the very thing they set out to improve.
On top of this, multi-tasking doesn’t even exercise task-switching as a skill. A study from Stanford reports that heavy multi-taskers are worse at choosing which task to focus on. (“They are suckers for irrelevancy”, as Cliff Nass, one of the researchers put it.) Multi-taskers often think they are like gym rats, bulking up their ability to juggle tasks, when in fact they are like alcoholics, degrading their abilities through over-consumption.