People take him seriously because he has a history of developing and creating (or at least being involved in the creation of) great ideas which require a lot of technical expertise, e.g. PayPal, Tesla, SolarCity, SpaceX, and Hyperloop. Besides, he went to Penn (Ivy League) and was accepted onto a highly competitive and scientific PhD programme at Stanford, so I suspect he has a lot of brain power and intellectual muscle which warrant his opinions and warnings being: a) news-worthy; and b) taken seriously.