You can make this exact same argument with caribou except that dogs are smaller and more used to being around people.
And horses and oxen. The only way to multiply horsepower was to put bigger teams together, up to four horses to a carriage. Still, the more animals you have to wrangle, the more difficult it is to wrangle. Do the math, it's no an irrelevant factor.
Or look up Goe's Animal traction guidelines, and start doing comparative calculations. I'm travelling, but if this thread continues I can probably find a link.
Aha, here we goe (pun)
http://eprints.icrisat.ac.in/8569/1/RP_00915_Animal_Traction....pdf
You want to focus on pages 13 to 37. Lots of interesting stuff there in terms of different animals.
One thing which Goe point out at page 13 is that efficiency declines the more animals you use in a team. They don't really go into the reasons for this, and it's confined to assessments within a species, not between species. The efficiency decline can be partially addressed, I think, with effective harnesses and management. But the reality is that the more animals you use, the more there is an efficiency cost.
Don't underestimate how expensive dogs are too feed, particularly in numbers. In small numbers, or in special environments like the arctic, you can make it with dog labour. But if you were to seek the numbers of dogs you'd need to match the horsepower of a grass eater, on a wide scale.... well, that would go far, far beyond the available surpluses.