Why do shells call fork()?

When you call an exec family method, it doesn't create a new process. Instead, exec replaces the current process memory and instruction set etc., with the process you want to run.

As an example: you want to run grep using exec

bash is a process (which has separate memory, address space). Now when you call exec(grep), exec will replace the current process's memory, address space, instruction set etc. with grep's data.

That means the bash process will no longer exist.

As a result, you can't get back to terminal after completing the grep command.

That's why exec family methods never return. You can't execute any code after exec; it is unreachable.

As per the pts, check it yourself: in a shell, run

echo $$ 

to know your process-id (PID), I have for example

echo $$
29296

Then run for example sleep 60 and then, in another terminal

(0)samsung-romano:~% ps -edao pid,ppid,tty,command | grep 29296 | grep -v grep
29296  2343 pts/11   zsh
29499 29296 pts/11   sleep 60

So no, in general you have the same tty associated to the process. (Note that this is your sleep because it has your shell as the parent).