Ask HN: Why extremely big companies acquire other companies in their own space?

On the surface it makes sense. Small company had something good so big company bought them as it'd be cheaper than to build in house. But I'm talking Google and Facebook big. To me acquisitions are signs of failure. Big company already has.. People: lots of them and highly skilled Money: to fund different ideas Customers: potential customers already use one of their products, it's a matter of cross sell .. which is easier than finding new ones. All it has to do is to build a product. Yet companies like to throw billions in acquisitions. Why? Am I missing something? Is it seen as cost of failure? Are they not logical decisions to begin with? Like if I already have the customers, manpower and know what they want why should I pay billions when I can have it built in 10th of cost