“Safe seats” are only really safe as long as rival political parties neglect to target them

Are so called “safe seats” safe because of the devotion of their electors to a particular political party, or because the other parties simply don’t bother wasting their resources on ‘unwinnable’ contests? Alun Wyburn-Powell points to several examples, both historical and recent, which suggest that an intimidatingly large majority may be more fragile than it looks, provided that efforts are made to contest the seat.