inter app data transfer especially
I'm not a developer, and thus will take your word for it until such time as I get a chance to look into it.
However, assuming your statement is correct, it is obvious that you fail understand their reasoning behind it and why limiting API access to other apps is actually quite a smart move.
Three reasons to start you off :
Privacy :
Prevent one app snooping on another (e.g. a rogue app snooping your emails)
Security:
Prevent one app from being able to launch attacks against another.
Stability:
Prevent one app from being able to destabilise others. E.g. The ability of another app to destabilise critical apps such as the "phone" app.
The more power you give to developers, the greater the risk they misuse it through poor programming practices in the rush to get their products to market. There are innumerable examples of this (look up "buffer overflows" for example).
Different apps (one has to buy quite a few apps to cover even basic requirements like decent email handling) work very differently, with different conventions, and different limitations.
We're talking about the core product here, not apps. I'm sure what you said above could be said of your android apps (or whatever third party software is called under android).