Because I’ve never used the speech feature of Mac OS X for anything serious* and 669 Mb for one voice, one voice, is just silly. It can’t be that good.
Just look at the monster http://www.jonathansaggau.com/blog/2007/10/yeah_it_breathes.html.
* I have, however, used it. My favourite use is ssh’ing in to a server that is located under the sofa in a friend’s living room and running
say "feed me!" a few times.