I always thought the reason cookers weren't isolated is because they normally have a clock which needs to be permanently powered to keep time.
The instructions for most showers however, say something like this example from Triton:
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As far as I can remember, none of the shower isolators I have changed have had loose connections, it has been the contacts that have overheated. When operating the shower isolator post showering the contacts won't actually have any current flowing through them at the time, so won't burn or pit from arching at this point. Therefore the damage must occur during the actual usage of the shower.