'C' systems never actually start from main().

main() is a function that has to be called from its environment.

With Keil, the very first thing that is executed from the reset vector is the startup.a51, or equivalent.
This then calls some other stuff that handles things like intialising variables, etc.
Finally, all this 'C' runtime startup calls your main() function.

However - none of this has anything to do with why you need inline assembler!
See: http://www.8052.com/forum/read.phtml?id=129336