Why do the F/A-18 and the F-22 Raptor have horizontal stabilizer as well as canted rudders for pitch control?
The rudders on modern fighters are canted mostly to reduce their radar cross section. A straight vertical tail would produce a corner reflector in combination with the fuselage or the wing and would send radar waves straight back to their source. To reduce the detection radius, such a behavior must be avoided. The first aircraft to use this trick was the SR-71.

On the F-18 an additional reason was to move the tips of the vertical tails into the lateral position of the chine vortex: The wing's big leading edge extension produces a strong vortex at high angle of attack which improves the effectiveness of the tails and gives better yaw control. However, the bursting of the vortex would cause heavy buffeting, and an additional fix was needed to improve the fatigue life of the tails.

The horizontal tail is a necessity for pitch control and to trim the flap moments in the landing configuration. Just look at Boeing's entry to the JSF competition: The X-32 was initially planned as a delta with only vertical tails, but when the Navy refined their carrier operation requirements, Boeing had to add a conventional horizontal tail to achieve the required angle of attack and minimum speed.



Boeing X-32 (left, source) and the planned production version (right, source) with the added horizontal tail. The F-117 was an Air Force plane with very limited agility, so it could get away with a tailless design. Real fighters and carrier airplanes cannot.

Giving the horizontal tail dihedral reduces its effectiveness in two ways: 


Angle of attack changes from pitch variations are reduced by the cosine of the dihedral angle, and 
the vertical component of the aerodynamic force is also reduced by the cosine of the dihedral angle.


Note that the drag of a V-tail depends on the total lift created, regardless of its direction. Only the horizontal component is effective for pitch control, so a steeply angled V-tail is very ineffective for pitch control. In air combat the high drag will reduce performance when it is needed most.