spoken like a true non-musician. or maybe a truly bored 21st century digital kid...
some people use it like you described. and many others don't. and music production/equipment is a multi (multi)-billion dollar industry, now being streamlined for [the much larger number of] casual musicians - who are LOVING getting into mobile music production. and people who do it on their computers are generally interested in doing it on their tablets. and $549 isn't very expensive, it's a couple days work for most professionals. and its resemblance to an iPhone is a tired talking point. and.... what else?
Might be a tired talking point, but it is still partly true. (Most musicians and DJs are rocking MacBook Pros with their setups, and running real pro tools software.)