It's because you questions were faulty. For example, you asked :
When, where, how and why am I wrong about a need for many national policies on agriculture?
The question is not, or should not be, if a national policy on agriculture is needed. It's whether or not it's constitutional. Even if I agreed 100% about the need for a Federal Department of Agriculture to regulate anything, and everything, to do with agriculture, that doesn't mean it's therefor constitutional. Proving the NEED for a federal agency is only a good argument for attempting to pass a constitutional amendment that would grant the power to tax the citizenry in order to provide funding for the federal agency that you insist is so needed. If that Amendment passed, that federal power would be every bit as constitutional as the Army, Navy, or to coin money.
Want to answer any of the questions I asked you? Or will you just go on insisting that if you think it's a good idea, it's therefor constitutional?