Because it is something that he would do, honestly. It seems like that is exactly what had happened when he possessed Alice's body
Somewhere down the road, Tiffany will transfer her soul into a younger body
Speaking of "outlive your usefulness", Tiffany may look beautiful now, but what about in 30 years? I doubt Chucky would want to sleep with an elderly woman. There's a high chance Chucky might kill her and get a new girlfriend in the future. Eventually she'll be too old and weak to assist him in any further crimes.
The next movie will have a small army of Chuckys
Mainly because this troper would love to see the lead one give this line:
(In his own natural voice, not the "Good Guy" voice
) Hi. We're
Chucky. Wanna play?