I still don't see why Yahoo didn't stick to their original purpose of human powered search instead of buying up random companies to try anything. All they had

I still don't see why Yahoo didn't stick to their original purpose of human powered search instead of buying up random companies to try anything. All they had to do was work out how to outsource the work to users. Why did they not buy StumbleUpon or PearlTrees, which few have heard of? That and email would have given them a lot of data on their user base.