I now see how you derived your definition of the wet zone. However, you cannot use the zone defined for lighting fixtures and apply it to switches. There is no Code support for your opinion.
Someone, Marcel, I think said he doesn't feel safe if you can touch something while standing in the tub or shower. Heck some bathrooms would need to have everything installed outside if that were the case.
If I were to try and discern a reason for the difference it would be because the fixtures have the potential to expose you to live parts whereas a switch would not.