Why is this?
Well, given you've provided so much information..... I'd say it's because you didn't dance naked under the Summer Solstice while burning money.
I can lock/unlock paths in symbols and AI 16.0.1 with no problem.
Not exactly sure what you are asking.
To edit the paths in a symbol you have to double click the symbol to entere the symbol, or you can break the link to the symbol.
To lock other paths than your current selection do a select >> inverse priro to locking. Or try object >> lock >> other layers
This is for CS5. However, you can lock layers in a Symbol. That is not the question. The question pertains to locking all other layers/unlock all layers.
In symbol edit
Lock other layers in the layer window menu does not work, it is greyed out.
Object>Lock>Other layers is also greyed out
Why is this? Combined with the fact there doesn't appear to be a way to select all objects on a layer, this makes reasonably editing symbols almost impossible.
Symbols only contain 1 top-level layer, then as many sublayers as you desire. Sublayers are not the same as top-level layers, therefore the lock all/other layers is not applicable. There is no "lock all sublayers" command.
I'm not seeing an issue with selecting all on a symbol sublayer in CS5 either. Option/Alt-click the circle to the right of the sublayer name.
Perhaps you need to reset preferences?