I wonder are you talking about the door liners rather than the frames? The frame is typically hidden from view, whereas the liner and architrave is what you actually see. There'd be little value to replacing the frames and it would involve some risk as others have said, whereas the liner/architraves are just lightly fixed onto the frame so can easily be changed if you want.
FWIW I replaced doors recently and wanted to modernise the look, so replaced the architrave but left the liners in-place for most of the doors as they were in good condition and don't tend to have a profile to them, they're just standard planed timber.