Why does the salutation end with a colon?

Why does the courteous close end with a comma?

The Arrangement

In the following, notice the spacing. If the heading is short, it is put on one line; as,

Heading Hilliard, Fla., June 30, 1914.
IntroductionMr. Thomas Barrett,
Boston, Mass.
SalutationDear Sir:
Body ..................................................
................................................................
.................................
Courteous closeYours truly,
SignatureSamuel Garth

If the heading is long, arrange it in one of the following ways:

1

334 Lexington Ave., Chicago,
May 19, 1915.

2