For your first question, either you know the concepts needed to program for iOS or Android or whatever, or you don't. If you don't, you likely won't be able to make a new app. If you do, learning the language will be easy. It will be harder to learn the frameworks and libraries, but you can do it.
For your second question, the multiplicity of environments came about for good reasons. Different companies start with different goals, and start at different times. Mac programming, for Mac OSX or iOS, originated with the NeXT cube something like twenty years ago or so (ever notice all the NS prefixes for NeXT Step?). At the time, Objective-C was a contender, and NeXT and Apple have put a whole lot of work into the environment over time.
Google came along much later, with much less baggage and probably somewhat different design goals, and wound up with something seriously different to work with. They weren't going to use Objective-C, because they thought Java a better choice.
The only ways iOS and Android would have had the same native language would have been if Apple had converted over twenty years of work into Java, for no real gain, or if Google had deliberately remained more compatible with iOS. Neither was going to happen. Apple, in particular, didn't become the most profitable computer and technology company in the world by trying to do what everybody else did a little bit better or a little bit cheaper.
Moreover, nobody's smart enough to set up the best possible development environment for a given application. People are smart enough to come up with different features, some of which will be better, and to adapt other people's features to their own system.
We aren't going to get convergence of platforms until we know what's best, and I don't think that's going to happen in my lifetime. Is Java better than Objective-C? Quite possibly, although they're very different languages. Is Java the best possible language for developing mobile apps? I very sincerely hope not. If we had a common platform, somebody coming along with a better idea would get an immense amount of resistance, far more than today, and software development would stagnate.