> Why is the dot/period character used as file extension separator?
Why not? Maybe it was realized that other candidate non-alphabetic symbols were already reserved for other things, like '/' as a path delimiter on Unices, '\' same purpose on Windows, '#' as a comment delimiter, '%' as the modulo operator, '|' as the command pipe symbol, '^' to signify either power or exclusive-or depending on context ... we're quickly running out of candidate symbols on an ASCII keyboard.
Why not? Maybe it was realized that other candidate non-alphabetic symbols were already reserved for other things, like '/' as a path delimiter on Unices, '\' same purpose on Windows, '#' as a comment delimiter, '%' as the modulo operator, '|' as the command pipe symbol, '^' to signify either power or exclusive-or depending on context ... we're quickly running out of candidate symbols on an ASCII keyboard.