It's hard because it's dependant of other systems that are often not fully functional. The things that needs to work for a video call is: The network must be r

It's hard because it's dependant of other systems that are often not fully functional. The things that needs to work for a video call is: The network must be reasonably reliable and not overloaded. The camera must be configured and not privacy blocked. The correct recording device must be used, it must be recording (opportunity for both user error and OS issues here). If hardware codes are used (a requirement on lower end devices) it must support low enough bitrates, must have the right options. Typically all these things are slightly different between different devices and operating systems. It's typically easy to build a proof of concept with great quality and reliable connections between two given devices over a given network. It's super hard to make a product that is reliable enough that millions of users only rarely run into issues.