why fork and exec are kept 2 seperate calls

The main reason is likely that the separation of the fork() and exec() steps allows arbitrary setup of the child environment to be done using other system calls. For example, you can:

• Set up an arbitrary set of open file descriptors;
• Alter the signal mask;
• Set the current working directory;
• Set the process group and/or session;
• Set the user, group and supplementary groups;
• Set hard and soft resource limits;

...and many more besides. If you were to combine these calls into a single spawn() call, it would have to have a very complex interface, to be able to encode all of these possible changes to the child's environment - and if you ever added a new setting, the interface would need to be changed. On the other hand, separate fork() and exec() steps allow you to use the ordinary system calls (open(), close(), dup(), fcntl(), ...) to manipulate the child's environment prior to the exec(). New functionality (eg. capset()) is easily supported.

fork and exec do completely different things.

• fork() - duplicates a process
• exec() - replaces a process

There's plenty of reasons to use one without the other. You can fork off child processes that perform tasks on behalf of your controlling parent app e.g., pretty common in the unix world. And you can e.g. setup the preconditions for some other quirky application and then exec it from your launcher application without ever using fork.

AFAIK, initially there was only fork(). But performance reasons called for creation of exec() that does not re-create the kernel structures that are going to be immediately overwritten. <-- This is wrong.

There's a performance problem when a process needs co create a child process which is not a copy of itself. A fork() copies process-related kernel data which are going to be immediately replaced by an exec(). Thus vfork() was introduced that does not copy excessive kernel data and any process data; after it, a process is expected to call something exec()-like, and the parent is suspended until the child does so. See 'Bugs' section for description of problems with vfork(), though.