There is a link to IBM Manuals at the top of this page. I STRONGLY recommend you find the COBOL
Language Reference manual and read it cover to cover. If you do this, you will find in section 1.4.1 this (emphasis added):
Period {.b},
Comma {,b}, Semicolon {;b} A separator comma is composed of a comma followed by a space. A separator period is composed of a period followed by a space. A separator semicolon is composed of a semicolon followed by a space.
The separator period must be used only to indicate the end of a sentence, or as shown in formats. The separator comma and separator semicolon can be used anywhere the separator space is used.
In the identification division, each paragraph must end with a separator period.
In the environment division, the SOURCE-COMPUTER, OBJECT-COMPUTER, SPECIAL-NAMES, and I-O-CONTROL paragraphs must each end with a separator period. In the FILE-CONTROL paragraph, each file-control entry must end with a separator period.
In the data division, file (FD), sort/merge file (SD), and data description entries must each end with a separator period.
In the procedure division, separator commas or separator semicolons can separate statements within a sentence and operands within a statement. Each sentence and each procedure must end with a separator period.
So the answer to your question is, "because COBOL syntax requires the period after the program name" -- end of discussion.