It is possible for an Instagram Model with a large following to make a career out of it. For instance, Jen Selter, a Fitness Model on Instagram with millions of followers have secured numerous sponsors for her page. That is not the case for most Instagram models, many just pose half naked pics of themselves hoping for likes and new followers. The fact that some people are unaware that it's possible to make a career out of posting pics on instagram is one of the main reason being an Instagram Model is seen as a waste of time.
Prior to the birth of Instagram, most individuals whom are currently on there, would have realized they are not model materials and move on to another career. However, due to the fact that there are many individuals (thirsty) constantly liking their images, it gives them the impression that there is still a chance for them to become a model; unbeknown to them, the older they get, the lower their chances of it happening.