Dogs don't care how vilely you behave, they are happy with the bare minimum, and they will love you no matter how badly you treat them.
Perfect child substitute for the lazy and selfish.
And although it does not need saying: yes, dogs are wonderful companions for decent people too.
Cost of a child can be $15,000 per year or $270,000 in 18 years (or half that in poorer rural area). And that is if you don't get taken to the cleaners with a divorce and child support.
From BabyCenter Cost of Raising Your Child Calculator
$130,680 for rural, two parents, no college, household income under $56,300/year, child born 2011.
$236,070 for suburb/city in midwest, two parents, no college, household income under $56,300 to $98,500/year, child born 2011.
From USDA Cost of Raising a Child Calculator
$164,250 for rural, two parents, no college, household income under $58,000/year, child born 2011, 18 year cost.
$263,934 for suburb/city in midwest, two parents, no college, household income under $57,400 to $99,390/year, child born 2011.
Pretty good that those people have dogs instead of kids.