Depth of Concrete Piers for A Deck
For an answer to this question, you have to ask yourself why they call for the deep footings on the house in the first place. The reason is that the freezing and thawing of the ground causes anything within the freeze boundry or layer to shift, move and heave upward. This caues unlevel houses. It will cause an unlevel deck too.
That's about it. If attached to your house, you don't want it to move around, causing stress on the connection area, cracking siding, twisting the deck, etc. And if the deck moves, when your house does not, those things can happen.
"True eloquence consists in saying all that is necessary, and only that which is."
François Duc de La Rochefoucauld