Cecil Moore wrote:
"What am I missing?"

Radio waves spread during propagation. Huygens` principle is a cause.

Huygens says:
Each point on a primary wave front can be considered as a new source of
a secondary spherical wave and that a secondary spherical wave front can
be constructed as the envelope of these secondary waves. This is
illustrated in Fig. 5-37 on page 144 of the 3rd edition of Kraus`
"Antennas".

Best regards, Richard Harrison, KB5WZI