They dont need to. But they should be in the goodybe trilogy for the old cast.
They are in the film, but from what has been reported they aren't in it that much. Having another film with Artoo & Threepio as the comic relief of the group would be a pretty big retread and I don't think they seem to be going for that to such a degree.
This should have be obvious with the fact Anthony Daniels was not part of any of the location shooting.
And think about it. You have to keep the main group somewhat small. We already know we have Rey (Daisy Ridley), Finn (John Boyega), Poe (Oscar Isaac), Han, Chewie, & BB-8 as the main group for the film. With Luke likely tagging along once they find him. That is already 7 people.
Compare to the other first films main hero groups who went on the main adventure
The Phantom Menace:
1. Qui-Gon
2. Obi-Wan
3. Padme
4. Anakin
5. Jar Jar
6. R2-D2 (Threepio was basically a cameo in this one)
A New Hope:
1. Luke
2. Leia
3. Han
4. Chewie
5. Obi-Wan
6. R2-D2
7. C-3PO
The Force Awakens:
1. Finn
2. Rey
3. Poe
4. Han
5. Chewie
6. Luke (Assuming he tags along once they find him)
7. BB-8
There really isn't room for 2 more to tag along.