I don't think those apps were just started with a boyout, acquihire, or an IPO in mind. But to "succeed" and be able to get money out of them. Do better in life, have a better (maybe a lot better) lifestyle and be able to support it for considerable time. Maybe a desire to retire younger. So when a good offer comes around they go for it. Because you never know - another excellent XYZ todo app from some ABC MNC or startup might have killed it in future - just an example.

This is normal, why would you be shocked by it? Consider going for open source apps only that supports open standards. That is not full proof and not always convenient but it's a good way to avoid some of thse shocks and horrors.