That's like saying the whole point of doing a Kirk & Spock story is to feature Shatner and Nimoy.
No, because they usually were already under contracts
with clauses to enable negotiations for each sequel. Note that, when ST:TMP was being written, there was no Spock in it because Nimoy had indicated he would not sign for "In Thy Image", and therefore Xon was created.
The OP asked why no Q in a TNG film. Sure, Paramount could
have ordered a TNG film with a role for Q that would be played by a different actor to de Lancie if he turned it down. I am assuming that the OP is asking for a Q film because he/she likes de Lancie's performance.
This is just going round and round in circles.
Mind you, there's certainly no shortage off ST 2009 detractors who continue to say that "the whole point of doing a Kirk & Spock story is to feature Shatner and Nimoy".
Good grief, you're right -- not that Q should've been in a movie, but that he essentially is Mr. Mxyzptlk in every major respect.
And you can make Q disappear by tricking him into saying his name backwards.