The hooking punches (so called "haymakers") you are describing are not the punches of a "trained" boxer/fighter. Yes, indeed, if you extend your arm fully out there, you can end up losing it, never mind making it easier to see and block or enter into. Also, there is nothing backing the fist when it hits. With a straight punch, direction of power comes/goes straight up the legs, hips, body, shoulder arm, wrist and fist. With the haymaker, the fist comes swinging around and there is nothing backing that fist, minimizing the power generated through legs, hips, etc., (that's not to say a person with a lot of mass can't generate knockout power with the arm/fist like that.) In fact, a good hook will have the arm in the "square" position, i.e elbow bent. On a good hook, the fist and elbow should travel pretty much along the same plane. The hook is initiated with a torque of the hips, followed milliseconds later by the bent arm/fist. Pretty much the whole body gets involved in the punch.
My words don't do justice to the mechanics of the hook. Best to look tapes of the old Floyd Patterson, Frazier, Duran or even Tyson fights. Where you throw the hook generally is closer to the opponent then the ideal range for the straight punch. If you look at distance continuum of hand strikes, it goes from jabs, crosses, hooks/uppercuts to elbows.
The saying "all is sanchin" is not totally accurate... The hook and body mechanics for it are not in sanchin.