This is short-sighted. A job is what you make of it -
you can get lots of different things out of it. Otherwise pro-bono work, internships, etc. couldn't exist. If all you're in it for is the paycheque in 2 weeks time, then that's all you're likely to get out of it.
When you look at a career as a whole - from both sides of the relationship - then it is beneficial for both you, and your employer, for you to increase the value that you provide to your employer. That can mean learning more so that you can be more efficient, do more (volume), do more (variety), do higher valued things (ie. via promotion).
Also, you're not wrong that you can learn in a big 'boring' corp but the conditions have to be right for it to happen, and those conditions are often more prevalent in companies that have knowledge or skills gaps - which is more often the case in smaller organizations (doesn't have to be a startup, could be a historically-under-funded/under-recruited IT group in a well-established corp).
