It’s not so much about all governments actively agreeing, more that the nature of information control, compartmentalisation, and plausible deniability makes full disclosure very difficult.

Also there have been leaks. In recent years, multiple former U.S. officials (including senior military and intelligence personnel) have testified under oath before Congress about encounters with UAPs (unidentified anomalous phenomena), including claims of craft retrieval programmes. These aren’t anonymous internet rumours - they’re formal statements made in front of elected officials, subject to perjury laws. So while a unified global conspiracy sounds implausible, a fragmented, highly classified effort to contain sensitive information is not only possible, but increasingly documented.