There is one reason that I can think of for why they would be classified as unhealthy - they distract you from what is happening right now and, as a result, you might miss something important. Multitasking is widely recognised as being a counter-productive activity. Do one thing at a time, with your full attention, and you will excel. Do many things at once and you will do all of them badly. Or one could say that multitasking is the process of failing at many things simultaneously.