Because some people buy in litters of puppies and sell them as though they have bred them, and will not necessarily know which puppy is from which litter, although they may well give you false paperwork. I know someone who worked at Norwich airport, and they had a load of about fifteen Westie puppies come from Ireland, all in one crate. They were delivered to a well known puppy farmer in the county and no-one could possible tell which puppy was from which litter.
Also you cannot tell the size that the pup is going to be or the temperament if you have not seen the mother.
Although some people say you should see the father too, this is not necessarily the case. I have travelled hundreds of miles to use a stud dog if it is the right one for my bitch, and although you could see the father at a show or pictures of him, unless you wanted to go hundreds of miles you would probably never see him. Also if people have both mother and father, it sometimes means they are just pets and they have mated them together, maybe without even knowing whether they were related, or having any health tests done.
Most decent stud dogs are only available to breeders who do all the health checks on their bitch.
Get in touch with the breed club and ask for details of reputable breeders who may have puppies available.
The Kennel Club also have something called the Accredited Breeder Scheme, which although not perfect goes some way to assuring that the breeder carries out health checks and provides proper information and paperwork.