Because the alternative is not resetting the state and having to deal with the fallout. iOS does this and it's a source of even more strange bugs and side issues (based on doing cross-platform development across several teams).
This even led to the point where a huge amount of iOS apps simply banned rotation support to not deal with it.
It's just a different tradeoff. The underlying issue is that developers don't want to deal with storing the state which causes issues on lower end devices, which start to break as users multitask. The Android tools for that also aren't all that great - although I also haven't really seen any other toolkit do it better.