Q.Why are dish antennas curved?

Because the law of reflection is obeyed at every point on a spherical mirror, the image of an object point can always be located graphically once any two of four standard construction rays are drawn from that point (their intersection, or the intersection of their backward extensions, is the image point). (i) A ray travelling parallel to the principal axis, after reflection, passes through (concave) or appears to come from (convex) the principal focus $F$. (ii) A ray travelling through (concave) or directed towards (convex) the principal focus, after reflection, travels parallel to the principal axis -- the reverse of rule (i), following the principle of reversibility. (iii) A ray travelling through the centre of curvature $C$ strikes the mirror normally (since $C$ lies on every normal) and retraces its own path after reflection. (iv) A ray falling on the pole $P$ reflects symmetrically about the principal axis, obeying $i=r$ with the axis itself acting as the local normal at the pole. Only two of these rays are needed to pin down where the image of any object point forms; a parabolic reflector such as a satellite dish or torch reflector is curved for exactly this reason -- to send every ray parallel to its axis through (or from) a single focal point.