Rescue dogs aren't for everyone and sometimes it's not suitable for someone's requirements, that's why.
If you are after a specific breed that is actually going to resemble that breed looks and size wise, you need to go to a breeder.
If you want to get into showing your dog later on, you need a pedigree dog from good lineage. Exceptionally good if you want to really get anywhere with it.
Different breeds of dog have different genetic conditions, if you want a dog who has come from a health tested line you have to go purebred, as it's the kennel club who run these health tests schemes and to be eligible, the dog must be registered with them.
Most people with young children would want a well socialised puppy that has been reared with kids from day 1 in the breeder's home, rather than a dog of unknown history, and many shelters won't allow a dog to go to a house with children under a certain age in it.
Some people prefer to get a well bred, health tested, well reared puppy, that has been given the best possible start in life with no expense spared to have the best chance of growing into a well adjusted healthy adult dog, and can be moulded into whatever the owner wants from the dog.
Not everyone can take on a rescue dog, many perfectly good homes are turned down by rescues because of their ridiculously strict box ticking. Not many people like being scrutinised by some jumped up busybodying little official, who makes you leap through hoops then gleefully deems you unworthy and not good enough

. That might seem like being OTT but it really isn't, I've spoken to many people who've said those in animal rescues are too strict for their own good. It'd be easier to break into the bank of England than pass some shelter's checklists

. They don't quite work like they show on TV.
Some owners want the fun of seeing their dog's puppy days so they can treasure them when the dog is older, in the same way parents don't want to miss out on their baby's early years. My dog was just six weeks old when I got her, I have so many delightful memories and hundreds of photos of those first weeks that I wouldn't change for anything. I really couldn't bear the thought of getting a dog at 1 or 2 or older and missing out on those wonderful early weeks, to me that's like having a baby then missing out the first few months by handing it to someone else, you'd lose out completely on those irreplaceable first months that can never be got back. That's just me though I'm sure there's people with the same or similar view, and there are others who it wouldn't bother in the slightest, and whom I admire as I couldn't do it. What suits one person doesn't suit another, everyone is different, has different requirements and wants, which is why it's important to have the choice of breeder or rescue.
Those are some of the main reasons for buying a pedigree dog from a breeder

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