Well if Celebi is any indicator, we could never see them.
I don't really have any definite ideas or theories about Reshiram and Zekrom, but Victini has something in common with Entralink Pokemon (which are also shiny-locked); it's injected into your game and placed in a location as an interactive Pokemon. Which makes me believe that any interactive Pokemon either spawned through outside injection or an event like Reshirams/Zekroms were either chosen to all be shiny-locked, or there is a coding issue with the way their PIDs are generated that prevents them from being shiny. Hidden Hollow Pokemon also are shiny-locked and are randomly generated interactive Pokemon.
Stationary legendaries are also interactive, but their spawns are different. Comparing them is like comparing a Pokemon fished from regular water and one from rippling pools.