I might be completely wrong, but Zimmer seems to collaborate extensively with his ghostwriters. If you look at the cue sheets, many of them are responsible for major cues and sometimes even thematic material.
Generally speaking, if you look at the BMI repertoire, Elfman is solely responsible for the majority of cues in his scores - with the apparent exception of Goosebumps and Avengers. And the cues he tends to farm out are generally (though not always) minor.
Halli Cauthery is credited for additional music for Hellboy II, and he wrote around a minute or so of music, for example. Lindgren got an additional music credit for Oz, and all he wrote was a short source piece that one of the characters plays for a few seconds. On the other hand, Bacon seems to have had a very large role with Goosebumps.
There's also the fact that Elfman needed no help on The Girl on the Train, End of the Tour, The Circle...which suggests that it's an issue of time. I remember an interview for End of the Tour where Elfman said one of the reasons he was so excited to be apart of it was because he could do it all himself.
T.J. Lindgren, who tends to help Elfman a lot, uploads his cues/arrangements to his soundcloud. They're generally minor, and in the case of Alice 2, never even made the album.