We've all been wondering why that dam button is there.
Why won't it work for this? Well first off the file size is something like 75-100mb in size. To large to download over the air. Secondly, the process for flashing a ROM is much more complex then simply install an app using a process as in a .cab file. Part of the process involves clearing the OS off the device leaving just the bootloader and little else. Not a process that lends itself to over the air. It is possible if you know the right procedure to flash a device from a file on a storage card, but by far the safest and easiest way is through a desktop.
The update button is for future use when there are small updates with small files that can be installed much like using a .cab file