Why is programming done in different languages for different platforms(OSs')?

Question

for example:-
Android-Java;
ios-objective c, Swift;
Windows-C++;
etc...
also can cross-platform programs be written?

Answer 1

For platform do you mean operating system ?

Answer 2

I didn't get it, what you want to know about ?
is it the environment for the individual language you want to know ?

Answer 3

os

Answer 4

why are iOS apps written in objective c and swift ? rather than Java or c#