It was a process geared specifically to be used for Rei (who has the soul of an Angel, not a human), and might not even work at all for anyone else. Misato and Shinji truly died in Chapter 1 and weren't able to be saved, and both have moved on. They're not coming back. Misako and Shinju, meanwhile, have completely new and different souls. How Ritsuko even accomplished this is not clear, but in Misako's case she made a reasonable facsimile of Misato. Shinju, on the other hand, is so unlike Shinji that it is very obvious that something is not right.