But nowdays you get them coated in a plastic layer ... very nice, water proof and durable.
edit: or like the othe commenter mentioned, it is not a plastic layer and rather plastic itself called Tyvek or similar
And in any case a map is a piece of safety equipment - you spec if for the worst case scenario.
NB When in Scottish mountains I take:
- An iPhone with offline OS maps
- An external battery for my iPhone and cable
- A paper OS map in a waterproof map case
- A printed A4 backup of an OS map in a plastic bag that I can give to others or to use if other maps become unusable (e.g. blown away - which has happened)
80% of the time I just use the iPhone - still have to use OS map and compass occasionally when things are particularly exciting.