Because if Android/Linux someday becomes they best developer platform (however unlikely), Apple will have real trouble. When the good developers loose interest in a platform it becomes very difficult for it to survive.
And good developers results in not just good apps. It's also recommendations to friends, support when something doesn't work or needs to be explained, and it's advertising that's much more powerful than the advertising you can buy.
All the money and ads Microsoft put into MS Phone did nothing because it didn't bring developers. Same with Blackberry and Nokia because those platforms were never developer friendly.