Because it's, like, really cool and stuff to add users through the sheer power of text file manipulation?
As I said before -- for the purpose of running Kindle hacks, there is zero utility value in adding a new user.
All the hacks run as root anyway, from the jailbreak script as a component of upstart, on through to KUAL.
Granted,
some things could be reworked for the sheer joy of using extra users, if anyone cared to bother.
KISS is what I say.