Because they can damage your phone and devices by not following proper lightning protocol. Over voltage, etcFor USB wall chargers, why pay the premium for Apple? I've bought tons of third party USB wall chargers from reputable companies and they cost a couple of quid. Sure, they don't look anything like the real Apple ones, but who cares?
I totally get it when it comes to buying MacBook chargers as they are proprietary and there's no way I'm going to risk frying a £2,000 laptop for the sake of saving a few quid on a charger, but for the plugs to charge your iPads, iPhones and iPods any old USB wall plug will do.