Fragment file PassThroughDepth.glsl can reused in PickDepth.js,why not?

[yieryi]

just like this topic, i find cesium/PassThroughDepth.glsl at main · CesiumGS/cesium · GitHub can reused in cesium/PickDepth.js at 34b0a1a245ddd4edfbf82c29e3c67baf019f7676 · CesiumGS/cesium · GitHub ,why not ? does there are some reason?