Why can not all lenses open up to f/1.0 or some wider aperture then is often available? It would be nice and certainly would help in low light conditions, having said that unless you had very short focal length lenses the depth of field may be shallow!
But lets look at what aperture really is. The aperture stated in stops or F number is a ratio between focal length and the size of the aperture opening relative to the focal length.
Essentially if we divide the aperture opening diameter by the lens focal length we get a number from which we can calculate the aperture. Lets say we have a lens with a focal length of 50 mm ( f = 50 ) and the aperture diameter is 25 mm. The math would then work out to a relative aperture size of 25 mm ÷ 50 mm = 0.5 or 1/2. This means the aperture opening is half the focal length. This is represented as an aperture of f/2, as when the math is worked out the focal length of 50 mm / 2 = 25 mm or aperture opening diameter.
This is why properly written the aperture should be represented with a notation like f/2 and not F2 or F/2, which we see so many times! The f in aperture equates to focal length of lens.
Now we know that lets step back a bit and look at the original question of the engineering challenges of f/1.0! To obtain that aperture opening in a 50mm lens the aperture opening must also be 50mm in diameter and added to that would be area to store the aperture leaves and any supporting mechanical parts on which the leaves are fastened and hinge. This would mean the outer body of the lens would be somewhat larger than the focal length, and significantly so in longer focal length lenses.
Now lenses of 50mm length do exist with widest aperture of f/1 and even wider! They are quite large lenses for a 50mm physically and are quite expensive to purchase but do exist.
However, lets take that to longer focal length lenses say 200mm or even longer 300 or 500mm lenses. Can you imagine a 500mm lens which has the physical size permitting the aperture to open to a 500mm diameter, room to store aperture leaves when fully open and any supporting mechanical structure to support and control the aperture leaves? The lens would be huge, perhaps 700mm or more in diameter! This is why it is more common on a lens of 500mm to have apertures of say 5.6 or even 6.3 as an example! So as we need manageable lens sizes which we can both afford to purchase as well as carry around, we are all going to see larger zoom lenses generally at larger numbers for widest aperture size!
This is also why zoom lenses which have even moderately low aperture sizes (say f/2.8) are quite large physically and generally fairly expensive to purchase.
There is also the question of a fixed aperture zoom versus a variable one, why the fixed aperture is better and why the fixed aperture costs more to purchase, but that is another blog.