Because "up to it" is an adjective
I don't know the Honda commercial, but I'm sure it doesn't say what you wrote. Did you mean "If your heart is up to it?" This is syntactically the same as "If your heart is strong enough". Or did you mean "Is your heart up to it?" This is syntactically the same as "Is your heart strong enough." "Strong" is an adjective, you wouldn't think of using "do" with it. (Well, at least I wouldn't). "up to it" is exactly the same part of speech.