There are only a few moving parts to the process.
Keywords, Ad copy, Website
By advertising to the exact keywords that they are using, generating copy that is comparable and a website that's competitive, maybe even better, how can you lose?
Worst case scenario you do a few tweaks here and there, but if there is an existing advertiser that's been doing the same thing for a good period of time, you can fairly safely assume there is money there and that they are fairly successful at extracting it.
What am I missing?