This is primarily for privacy and security reasons. One wouldn't want a funny casino app, or the likes of Google or Facebook to hijack people's web access.
Another aspect is the user experience - a piece of web content will render exactly the same way anywhere on the system. There is no need for people to be concerned with webapps asking absurd things like "Download browser X to view this page".
A third aspect is actually very helpful to developers - you can count on 100% of the APIs you intent to use being available on the system's web browser, creating in-app experiences significantly easier (and cheaper) to develop.