Q: Why doesn't
A: There is a very real difference between characters and strings, and strcat concatenates strings.
A character constant like '!' represents a single character. A string literal between double quotes usually represents multiple characters. A string literal like "!" seems to represent a single character, but it actually contains two: the ! you requested, and the \0 which terminates all strings in C.
Characters in C are represented by small integers corresponding to their character set values (see also question 8.6). Strings are represented by arrays of characters; you usually manipulate a pointer to the first character of the array. It is never correct to use one when the other is expected. To append a ! to a string, use
See also questions 1.32, 7.2, and 16.6.
CT&P Sec. 1.5 pp. 9-10