Sure, those customized containers were nice in FSX or P3D but, there was a cost to them, and it was adding ( as of today, it will only increase when new operators are introduced ) 3587 new Simobjects in the list of the objects available in the simulator.
This has a great impact on the simulator loading time and, while P3D still loads reasonably fast even with all those objects added ( although it's way faster in P3D 4, were we had Render To Texture with DirectX, so the ULD were just 11 ), considering how longer it takes to start MSFS, we don't want to be the bad guys single-handedly contributing the most to a slow startup time.
Of course, should MS/Asobo ever to add our suggestion to have any kind of Render to Texture feature, which will offer benefits far more widespread than just that:
https://devsupport.flightsimulator.com/idea/349/render-to-texture-features-loading-gauges-on-any-o.htmlWe'll be the first jumping on the feature.