Why do we say "Hurricane prepares to hit east coast?"

Why do we say "Hurricane prepares to hit east coast?" People prepare for a hurricane, but the hurricane isn't doing anything but moving.

When else do we say that an inanimate object prepares to do something?

drɱ65 δ's answer is correct in that the usage of the word "prepare" is metaphorical. It is perhaps easier to understand the metaphor if you look at how a hurricane develops.

A hurricane is formed out in the deep ocean as a result of several complicated meteorological factors. You often hear hurricanes being categorized as Category 3, Category 4, etc. on the television. This is a 1-5 scale that categorizes the hurricane by its strength. A hurricane keeps growing in strength as it brews out in the deep ocean (as long as the conditions are favourable) and every hurricane goes through categories 1 until whatever the number is when it hits landfall. Landfall is actually when it starts to lose its strength and fizzle out.

If you put yourself in the hurricane's shoes, you could perhaps consider the entire build-up phase in the ocean as preparing itself for an attack on the coast, reaching a crescendo at landfall, and wreaking havoc quickly. Sort of like slowly cocking (preparing) a giant catapult in the medieval ages and ending with a quick shot that potentially caused damage to the other side.

Perhaps the prevalence of its usage among weathermen is because they've been tracking its progress for longer than you've known about it, and to them, it feels perfectly natural that the hurricane is preparing for causing large scale destruction.

This is a metaphorical usage of the word prepare. Naturally the hurricane does not plan or get ready for anything; but the meaning is still clear: Hurricane will soon hit east coast. The metaphor can be used to enhance the statement by assigning the hurricane humanoid emotion/mentality, a very old tack in human history. A term for this kind of depiction is anthropomorphic, as in an anthropomorphic rendition.

The use of metaphors like this is not at all uncommon, and this usage is perfectly acceptable.