High quality speech recognition is ubiquitous in phone services these days so that isn't a good reason. Besides, if I want to set a 16 char password and am willing to enter that on a keypad, what's the problem?
Don't worry, you can enter 16 characters, but it only will care about the first 8, everything else can be random when you log in.
Also, if you have an a, b, c, or 2 in your first character, you can use any of them in place of that character index to log in.