Because 3 sigma doesn't mean 99% chance they've found it; it means that, if it doesn't exist, there's a 1% chance of obtaining the results they did. Common statistical misconception.
Consider a situation where they test 1000 hypotheses, only 100 of which are actually true. Of the 900 false hypotheses, 9 will, by chance, achieve a 3 sigma "99% chance of being right" result. And hence something like 8.3% of the hypotheses tested as "true" will be false positives -- not 1%.
Consider a situation where they test 1000 hypotheses, only 100 of which are actually true. Of the 900 false hypotheses, 9 will, by chance, achieve a 3 sigma "99% chance of being right" result. And hence something like 8.3% of the hypotheses tested as "true" will be false positives -- not 1%.