It is because the termination of them is all different.
I.E. If you look at a resistor, it has metalic coating around the end only yet a capacitor may have it surrounding the complete end (top/bot, end, sides)
Inductors too have a different termination than say resistors.
So they need different solder profiles, different fillets, side fillets not just end etc.
Then there are different soldering methods, wave, reflow etc.
Then you have different densities, smaller footprints because the board is having more stuffed onto it, its higher volume, perhaps larger footprints for higher reliability (mil spec/medical).
Then you have different engineers views on it.
Then your piece of string has different lengths.
Hence different footprints.