Actually they can, and frequently do. In fact, it is highly likely that is exactly what happened in the case of this movie. It was largely shot (supposedly) and suddenly got pushed way back, and only after that delay did we get told it was sequel. Judging by the timing, I'm sure initial reactions from both studio and test audiences were unimpressive. Complaints from test audiences and studio probably included copious complaints about how little it had to do with the books. Thus begins more shooting and edits to try and salvage. Part of that process was to try and nip the negative reaction to the unfaithfulness to the source material by deciding it was a sequel.
*If you want more anecdotal evidence of that, consider that fabled Horn, upon which rests the entire notion that this was the sequel and the last time around. You will find it has NO importance in the film. In fact, it doesn't even appear in the film. There are numerous attempts to try and figure out if it might be in Roland's bag somewhere though.
In fact, there is absolutely NOTHING in the film whatsoever that hints, states, or indicates that there was ever another turn of the wheel or that this is anything but a self-contained story. The only way we, the audience, know this is a sequel to the books is because they suddenly started telling us that out of the blue. The horn is not blown. This isn't a sequel. It isn't a pre-equal. It isn't an adaptation. It is just a mediocre to bad movie.
I'd watch it streaming, included in some subscription first before you drop the money. You might not want to own it.
And yet their fame did absolutely NOTHING for the film. Food for thought.