The biggest hard limit is optics: the f-number is focal length divided by entrance-pupil diameter. So at 600mm, an f/2 lens needs an entrance pupil about 300mm wide. That implies an enormous front element, huge glass, high weight, and very difficult manufacturing tolerances.

A zoom makes this much harder. A 5–600mm lens would need to work over an extreme focal-length range while keeping that large aperture, which means many large elements moving precisely relative to each other. As zoom range increases, correcting aberrations across the whole range becomes much more complex.

Large apertures also make optical flaws more obvious, so the lens design must control aberrations very tightly. Bigger elements are harder to shape, polish, coat, align, and support. The result is a lens that would be extremely large, heavy, and expensive.

So the first limits are mainly physical optics and engineering: required entrance-pupil size, aberration correction over a huge zoom range, and the practical difficulty of making and aligning very large glass elements. Economics matters too, but even before that, the lens quickly becomes impractically big and complex.