There are many reasons to work for a startup. Some have been downsized from large corporations and are looking to transition into a different career, while others simply prefer to run their own shop and avoid the bureaucracy of large & middle market firms. 
Regardless, there are considerable employment opportunities, since economic growth has been largely fueled by the startup and small business sectors, which frequently need the know-how of experienced executives and the dynamism of young managers who wish to build a firm from the ground up. As a consequence, major recruiting firms have become increasingly involved with startup and small businesses, given the significant role these sectors play.