Because Apple didn't do the wrong thing. The people just plugging their Macbook into a cinema projector and expecting their general-purpose desktop OS to work just like a cinema projector controller by default are doing the wrong thing. They simply don't understand what's going on.
As repeatedly said by many people, Apple is making a tradeoff between security and special-purpose requirements. If you have a special-purpose requirement, then make sure your tools are configured for that purpose. Don't whine that the general use-case doesn't fit it!
You'll see similar whining from web devs trying to make Chrome or Firefox "hide" the chrome and go full screen programmatically. That's fundamentally incompatible with end-user security and shouldn't be allowed, ever. Asking for it won't get you anywhere. If you need full screen control, use something like Electron, not a web browser. If you need full screen projector/screen output, use a tool/software appropriate for the job.
As repeatedly said by many people, Apple is making a tradeoff between security and special-purpose requirements. If you have a special-purpose requirement, then make sure your tools are configured for that purpose. Don't whine that the general use-case doesn't fit it!
You'll see similar whining from web devs trying to make Chrome or Firefox "hide" the chrome and go full screen programmatically. That's fundamentally incompatible with end-user security and shouldn't be allowed, ever. Asking for it won't get you anywhere. If you need full screen control, use something like Electron, not a web browser. If you need full screen projector/screen output, use a tool/software appropriate for the job.