Actually, its not. First, the amount of work in building an iso image is pretty much the same as building a usb key. Second, when you build a usb key you end up replacing isolinux with syslinux setup operates differently. Third, there's additional work required to enable setup to find the install files, which is more work then just incanting the correct command line once I know that command line.
Oh, I've tried to create a usb and when I try an install, is always says "can't access cdrom: