Doesn't make much sense to Morrigan to come back, either...
(Spoilers)
For story reasons. She ends up
a) gone to west if you reject the ritual

gone to west pregnant with a normal child if you romance her but reject the ritual
c) gone to west pregnant with god-child
(I'm not aware if there's another ending for her)
and d) It would destroy her personality ("You know what, I like... totally changed my mind, let's get married and have more babies, ok?").
And some practical, real world decisions:
a) They might not be able to sign Claudia Black to reprise the role in such short notice (unless they were thinking of this exp. WAY ahead of time).

Or maybe EA does not want to shell out more $$ to hire the big actors just for a exp, etc...
The other problem is that for Morrigan to be important to the overall story it's only if the dark ritual is set as canon, but both Bioware and a lot of players want to keep the canon 'flexible' by using your previous decisions.