() runs commands in the subshell, so by exit you are exiting from subshell and returning to the parent shell.
Use braces {} if you want to run commands in the current shell.

From bash manual:

(list) list is executed in a subshell environment. Variable assignments and builtin commands that affect the shell's environment do not remain in effect after the command completes. The return status is the exit status of list.

{ list; } list is simply executed in the current shell environment. list must be terminated with a newline or semicolon. This is known as a group command. The return status is the exit status of list. Note that unlike the metacharacters ( and ), { and } are reserved words and must occur where a reserved word is permitted to be recognized. Since they do not cause a word break, they must be separated from list by whitespace or another shell metacharacter.

Reference: Why does (exit 1) not exit the script?