Maybe many reasons,but I put it down to being an irresponsible pet owner! As you so rightly state,millions of unwanted pets are euthanized each year due to irresposible people who choose to breed their pets.
The over-population of unwanted companion animals is caused primarily by irresponsible humans. . If people practiced responsible companion animal keeping, including spaying or neutering their animals, the number of unwanted dogs and cats who are killed would decrease significantly.
Spaying/neutering our pets is part of the solution to pet overpopulation. Backyard breeding is part of the problem!!!
Reading some of the answers on here,I'm not suprised one bit why we have such an enormous amount of pets in shelters/destroyed!! "I only breed every two/three years" !!someone said. Does that not count?
" Because I have a champion dog"! said another.! So do I,his Mother won crufts,but I still chose to neuter him, rather than add to pet overpopulation!!!
"Has passed all neccessary health testing? I will also NEVER breed a litter unless I am breeding in hopes of keeping my next show prospect.".Isn't this still breeding?!!
As for getting a dog from a shelter, I have had three dogs from shelters,and ALL were wonderful dogs!!