Frakes had moved on as a director in 2001-02, with Clockstoppers
which was, as far as I remember, a modest success. He might have turned down the assignment of another Trek film if offered, if the Nemesis
script was as ill-judged as it seems to have been at any stage of its gestation.
I wonder whether Frakes deliberately stayed away from the writing side, unlike the other movie-directing cast members Nimoy and Shatner. Or perhaps he wanted to contribute but wasn't able because his influence wasn't the equal of the other actors who were given Trek feature writing credits (Nimoy, Shatner, Spiner) or otherwise made script decisions (Stewart).