The investment into a particular OS or ecosystem is not just one of money, but also time and experience. It takes time and effort to learn how to manipulate the systems in a way that makes them work in the way that you want them to work. It takes time and effort to learn which programs/apps are a good fit and how to use them. It takes time to become comfortable in a system.
The money for apps is also a large factor, in spite of the surface argument discounting it as important. $100 dollars worth of apps gets compounded wth the prospect of having to buy yet another $100 worth of apps to replace the abandoned ones. Not everyone who buys a tablet has a ton of disposable income. If it took months to save the money for the hardware then the idea of spending even more money to purchase apps equivalent to the ones which you have already purchased is a large consideration.
In the end the real argument against switching is this: why spend money and time to learn something new when it does not considerably improve upon the old, which works quite nicely? I have met next to no one who, having used multiple systems, honestly feels that ipads/iphones or android tablets/phones are all that much different from each other. Of course windows tablets nd computers are a whole different ball game, but that is not what we are talking about. We are discussing mobile systems and why people decide on one system over another.