If you (a Joe Schmo nobody) were able to get an invite, you really think a media company with millions of dollars of funding wouldn’t be able to do the same? They could easily get moles into every single private trackers, complete with full backgrounds to pass the interview process. Private trackers aren’t preferred because they’re inherently more secure. At best, that is only security theater, the same as the TSA. They’re preferred because enforced seeding rules, verified uploaders, etc ensures their torrents are healthy and helps prevent “video.mp4.exe” types of malware uploads.
Plus most people use a mix of public and private trackers. Private trackers are obviously preferred, but sometimes you don’t want to kill your existing ratio with a massive download that will take weeks to seed back up to 1.0.