Why do cops seem to get mad when they don't find drugs?

They seemed mad is nothing more than a personal interpretation

You were not arrested, detained or anything else, so what is your complaint

And the police did not make contact with you because they drew your name out of a hat, so what is the rest of the story here because you are not telling it all

I never saw an officer get mad. I have seen people think the officer is mad, though. 

Police forces budget has been going up for the last several years, at a rate that is much higher than inflation; all the while criminality was going down (as it had since oil companies stopped putting lead in gasoline).

Cops are the only people forcibly exposed to lead on a regular basis, since they have to practice shooting. That makes them likely trying to justify their existence at the same time as not being able to do rationally.

Yes I have noticed that when the police search you and they find no drugs nothing wrong they get all upset like you have done something wrong they get angry with you.

They mostly don't find anything wrong and have to justify why they are there. If they can fit you up they will.