In a nutshell, aperture value is the ratio of the focal length and the lens diameter. ie, A = F / D. For a 300mm lens at 5.6, the diameter D = F/A = 300/5.6 = 53.6mm. Imagine for a f1.0 lens, the lens diameter would be 300/1 = 300mm ! Can you imagine holding a metal tube 300mm in diameter, with maybe 10-12 pieces of heavy optical glass in it ?
Another problem is that the bigger the lens element is, the harder it is to manufacture with minimum defects. Therefore, big lens elements would be prohibitively expensive. Eg, the Canon 1200/5.6L lens would have a diameter of 214mm. Guess what ? It costs around US$100,000 ( +-20,000) , and has to be made to order. So your hypothetical 300mm diameter lens would cost even more !
Apart from straight aperture Vs diameter factors as discussed above there are a number of optical problems involved in producing very fast optics. Coma and astigmatism become very problematic and it requires a great deal of correction to make a fast lens with a flat field, low coma and astigmatism as well as low levels of flare, chromatic abberations (colours mis-focusing etc).
Very fast lenses also require the use of extremely specialised and expensive glass compounds as well as special optical designs to sort out the problems mentioned above. Add to this the mechanical requirements and tolerances required and the cost adds up fast.
It is worth nothing that lenses (below f1.0) are available from the likes of Rodenstock for TV use and have been adapted by a few photographers for use on SLR bodies.
Can anyone enlight me about as why at 300mm, a lens can only at most do at F4 or 5.6???
The standard professional 300mm lens is F2.8. Nikon made a 300mm f2.0 which was a special order (around 350 were made, the cost was a paultry 29,500 USD in 1984 which was just on 6 times the cost of the 300/2.8. The 300/2 is a stunning lens which has never been bettered optically, though I must admit that it's a bit large and heavy these days to be my preferred 300mm unless I need exceptional quality.