This move saves money, since the servers process queries more efficiently-- “Right off the bat we can chop the number of servers that we use in half,” Burger says.

"Just improving their search engine" isn't some simple task. Google has a head-start measured in thousands of man-years. Closing that gap takes a great number of smart people a great amount of time.

I assume that Google is improving slower than Bing, since their algorithms and systems are more mature and closer to the "asymptotically ideal search engine".