Probably because they can see what happened with PCs happening again. If they don't fight android with everything they have now then in 5 or 10 years they might not have the chance.
Their market share is already going away progressively. There's way more Android devices being sold than Apple ones nowadays. They still have a clear edge on tablets, but for how long ? I can't see how Apple's growth can be sustainable unless they come on new segments (just like they did with the iPhone and the iPad in the first place). Fighting to the death on existing segments is not going to cut it in the long term.
All of this assumes that Android is Windows and that another open source platform, such as Firefox OS or Ubuntu Touch, wouldn't be available to pose any challenge to Android. The mobile sphere is probably dynamic enough to support such changes.
If the FOSS projects in mobile work hard enough now, they can eat into Android's market share, at least in the developing world. We shouldn't consider Android's position to be unshakeable.