From the little I’ve read there was no real evidence that a theft was committed. Apparently the judgement was based solely on the similarities of Perry’s work with the plaintiff’s. If so, this could be a wrongful judgement based upon an amazing coincidence, such as the rapper whose 2001 calendar showed a plane going into the twin towers, or the author who ten years before the Titanic sunk wrote of the large passenger ship Titan hitting an iceberg and sinking.
Why would it take more than that? If Katy Perry says she didn't steal it, and someone who created something they thought she did steal it...what other way to resolve it than to have a jury of people listen to both versions and decide?
I have never listened to any of her music, and have never heard anything from the other artist, but I'll bet I could listen to it and decide. A jury apparently did.
Honestly, what would be the alternative? Just a shrug of the shoulders and tell the complaining artist "Sorry."
As for coincidence, sure. It is possible.
Granted, there have been people who were worried that all the music that could ever be created had already been written, because there are only so many notes available.
Of course, that was back in the mid-18th century...