If you ran it for 400 cylces the reagent would be so diluted and full of garbage that you could find anything you wanted.
The test is run in a closed tube. You can't dilute a reagent mixture in a closed tube. Regardless of how many cycles you run the reaction, the contents of the tube do not change. How the contents are arranged may change--a mixture of loose nucleotides can be changed into a string of connected nucleotides in a positive reaction, but the overall content of salts, nucleotides, enzyme, and water doesn't change. It can't. The tube is sealed.
To use current vernacular: tell me you've never done PCR without telling me you've never done PCR.