Well, the first and most obvious reason is, because you need a place to live?
There are some other, less snarky reasons, as well.
From Real Estate Journal:
Below are several benefits of home shopping in a cooling real estate market — the silver lining to news predicting the residential real estate party is over.
1) More Selection
2) More room to negotiate
3) Interest rates are still relatively low
4) A home is still a good investment
Complete story: The Benefits of Buying a Home In a Cooling Real-Estate Market – By Amy Hoak, Real Estate Journal