Why does the stick force per-g test require it to be harder for pilots to pull back on the yoke instead of easier?
No because if stick force per G is too low, when maneuvering it becomes too easy to pull a lot of G and that's bad. It's like having power brakes in your car that don't build up more resistance in the pedal the harder you brake; it becomes difficult to regulate braking effort and too easy to lock the brakes (assuming no ABS).   Like with brakes, it's desirable to have the stick force build smoothly  with G force, because this provides good tactile feed back that helps you regulate how hard to pull.

If the force gradient is too shallow, too little effort is required to pull G and if above maneuvering speed it becomes too easy to pull the wings off by yanking on the stick too hard, and if below maneuvering speed, too easy to enter an accelerated stall while maneuvering.

It's an interesting point you brought up because there is a very popular 2-place cabin homebuilt airplane called the Wittman Tailwind, that's been around since the 50s. A very speedy, efficient airplane, designed by a famous air racer.  It has a very shallow stick force gradient, which builds to only about 5 lbs at limit load (I'm recalling from an article from the 80s).  This means if you are above maneuvering speed, it only takes a 6 or 7 pound pull on the stick to possibly pull the wings off, or at minimum bend something.  Wittman designed in the shallow force gradient on purpose because he was a pylon racer and really liked very light controls including pitch, more or less for similar reasons as you described.  

However the Tailwind is not for the ham-fisted.  A pilot that is used to the airplane and had a light touch would be fine with it and would enjoy the very low stick forces, but for someone not adapted to it, it's dangerous.  Most airplanes have stick forces that build to 20-40 pounds or more, but you are never pulling that force in anything but aerobatic maneuvers.  Most normal maneuvering requires a 5 or 10 pound pull, and you're only holding it for a short time.

Transport airplanes with hydraulic controls get their stick force gradients by artificial feel devices that mimic the feedback forces that you would feel from airloads if the controls were manual, with loads that increase with speed and G.  I dimly recall that on the CRJ airliners the feel force for pitch was limited to 50 lbs and I believe most transport airplanes are similar.