There's little reason why that couldn't be completely automated. The car knows the outside temperature. The car knows the inside temperature and can easily know the humidity. Its pretty basic math to figure out if the window is going to fog up or not.
You shouldn't need to engage your defogger, your windows should just never fog in the first place.
I do agree though, most cars sold today don't bother automating this and that critical safety controls should be controllable by the driver. I think defrost controls should be controllable on the same stalk that lets you control the windshield wipers, its a pretty similar concept of controlling outside visibility, even if there automatic systems in place to prevent fogging. I don't think it should necessarily be at home on the center console seeing as how its an important safety control a driver should be in charge of and should take precedence over any climate setting.
None of these things are things you need to be doing while actively driving.