Because the Great Firewall operates as a blacklist and is only intended to prevent commoners from learning censored information.
If Zoom meetings where anyone can join became a popular source of unfiltered news, they'd either ban Zoom or require that a two-class system is set up to prevent ordinary Chinese users from using Zoom to communicate internationally, similarly to how Douyin (within China) and Tiktok (everywhere else) are two separate silos.
But people from other countries connecting to servers in China is not something the Great Firewall tries to interfere with.
If Zoom meetings where anyone can join became a popular source of unfiltered news, they'd either ban Zoom or require that a two-class system is set up to prevent ordinary Chinese users from using Zoom to communicate internationally, similarly to how Douyin (within China) and Tiktok (everywhere else) are two separate silos.
But people from other countries connecting to servers in China is not something the Great Firewall tries to interfere with.