Because scheduled tasks are a pain to administrate. We use psshutdown (in conjunction with psexec if I remember correctly from the last time I looked at the script) from the pstools package executed by one of the DC's in its spare time at night followed by a magic packet in the morning to bring everything back around - and I'll bet that if you look under the hood of the package, you find a fancy psshutdown frontend. All in all, it took one admin an 8 hour day to write all the code (VBscript and some C# to do fancy automation tasks in a much quicker fashion than VBscript alone does) and we did a limited test the very night it was proposed.
Thing is, these things are always sold by vendors directly to executive management - who proceeds to tell IT "We're buying it" and nobody in IT gives enough of a shit about the business to propose the obvious, free solution.