Infinite zoom is not possible in any app.
All apps have to cap the zoom somewhere. 6400% is a decent cap to use. I'd guess it was merely an arbitrary decision on the part of the developers/Adobe. I don't think there's any real reason why it's 6400% other than it's a nice round number.
When looking at the defaults, the percentages merely double - 100%, 200%, 400%, 800%, 1600%, 3200%, and finally 6400%. They probably could have gone to 12800% but that seems like a considerably big jump.
The problem is, when working on a 15x15 pixel image 6400% may not be enough. But when working on a 36x36 inch poster, 6400% is plenty. So.. it's all relative.
To zoom further, you could increase the size of the art. IF that's not possible you can use third party apps which zoom the screen rather than relying on Illustrator to go beyond 6400%. For example, on the Macintosh you can turn on zoom in the system preferences (accessibility) and that allows you to zoom the screen. This in conjunction with Illustrator's zoom will let you go far beyond 6400%.