Like the others have said, ADR (automated dialogue replacement) is common in most movies: lines may be rewritten, or the filmmakers realize that the mics didn't pick some dialogue up well enough, or the director wants some lines delivered in a different way than they were given on-set... In "Seagalogy," the author cites an interview with writer Joe Halpin, who claims that the entire premise of the film was changed during post production, which explains the sheer amount of ADR in this one.
Why is Seagal not doing re-dubbing his own lines? That's one of the big questions for his DTV career. Maybe he refused, maybe he wasn't asked. In the same chapter of "Seagalogy," Joe Halpin claims that Seagal was "disappointed" with the movie and wasn't aware of the extent of the changes to the storyline, so I'm going to guess he wasn't asked back in this case.