Because they use technologies that are more expensive for marketing, sales and "image" rather than what is actually best for the consumer and the device?
Unless GT AT and Apple have some secret manufacturing technique they're not sharing with anyone else in the world, the concerns that people are raising against sapphire screen for the iphone aren't unjust.
As others have said, Apple wouldn't introduce a new manufacturing method unless it really was an improvement on the previous one, so I wouldn't call it marketing only. As always, we shall find out if all this pays off when the time is right.