Wickard's use was not purely personal. He grew more wheat than he used, and he sold that excess on the interstate market (or, from a different viewpoint: he sold wheat up to the allowable limit and gave the rest to his livestock). This directly impacted the interstate commerce of wheat, because growing his own wheat meant there was less demand for wheat from other farmers. Scaled up to an entire nation, this would have rendered the wheat control law toothless. Generally, this law has been upheld consistently in this context, though SCOTUS has struck down cases where a good was tangentially related to Congress but the targeted act/good was not actually a commercial transaction (or the avoidance of a commercial transaction, as in Wickard).