It's funny you should say that. Because that's not at all true. You would know that if you tried to run XMMS today. Or an ages old flash player binary. Or Acrobat reader.
What is true is that nobody has wanted this compatibility so nobody has provided it. Because, as it turns out, it's easier to forward-port the applications that we have than to provide backwards compatibility. Because we have the sources. And the platforms you cite don't.