Why Modern antenna modeling software (NEC, MININEC, EZNEC, 4nec2) is built on the Method of Moments, a numerical technique that breaks the antenna into many short segments, treats the current on each segment as an unknown, and solves the resulting system of simultaneous equations for the currents using the mutual impedances between segments. Once the current distribution is known, the program computes feed point impedance, gain and the azimuth and elevation patterns. It is a numerical approximation, so results depend on using enough segments and on modeling the wires and ground realistically.
Watch out Mutual impedance between elements is one ingredient inside the Method of Moments calculation, not the name of the analysis method itself; graphical and calculus-based approaches are not what antenna modeling programs use.
NEC = Numerical Electromagnetics Code = Method of Moments: chop the wire into segments, solve for the currents.