Just like cold viruses, parasites and opportunists like fleas and ticks, are everywhere. They are on the ground, on blades of grass, on people's hands and shoes that come into your home, on the vet floors and tables, etc.
Why are puppies more likely to have them? Because puppies are together in a bunch. One will pee or poo, another will step in it (no matter how clean a breeder is, this is definitely going to happen with a litter), then the pup decides to clean his paws off with his mouth - bam, passed on a little friendly. Or, they'll step in something good outside on the ground, then come in to their pen and play and wrestle and step in their water bowl. That branch or leaf your cute puppy has in her mouth? Yep, has coccidia on it. That mulch your puppy keeps digging up and chewing on? Yep, has roundworms in it. That puddle or stream that your pup took a drink of while out on a hike? Yep, has giardia in it.
It's going to happen! And, they will pass it on to your other pets and maybe even you or your kids.
Sometimes, you will not see symptoms AND fecal analysis will return a false negative. So, they could live with an opportunist forever, and you might never know.
If that is the case, why should we try to control it? Because some can get quite sick from having parasites. Also, it could give them a sore tummy and make it hard for potty training. They also can pass things along to kids - and vice versa. Plus, treatment is relatively easy - so why not just try and control it.
In addition to treatment, clean all toys and bedding and bowls with soap and bleach - frequently. Whether you think your dog has any fun critters or not, you should do this every week at least to negate the chances of harboring opportunists.
Also, use monthly heartwormer that your vet prescribes. It not only protects against heartworm, it also has dewormer for parasites baked into it. Also, use a flea and tick med that covers your dog BEFORE a tick bites (right now, the current product to use is Advantix, not Advantage or Frontline); and consider the Seresto collar during the summer months for additional protection.
And - yes, I am biased here - it does not mean that your puppy came from a "bad" or dirty breeder. Not at all. I would like to know if there has ever been a breeder that has never had any parasites within their pups because I would say it's impossible. Puppies are just silly - they love getting into things, chewing on dirt or grass, and then stepping in bowls and mouthing at their siblings. Unless you want your puppy to be in a bubble, it's going to happen. And, as a breeder, I don't think you're a bad owner if your dog gets a parasites - no, not at all - because I know it's going to happen.
It is a fact and an expense you have to account for if you want a dog in your life.