In Avengers: Endgame Bruce and Iron Man
snap for the first time
yet they don't visit the
metaphysical realm like Thanos did where he meets Gamora.
Why is that so?
Out of universe, the directors planned to have a scene in the metaphysical realm with Tony and an older version of his daughter Morgan:
Apparently, Langford was going to play the older version of Tony’s daughter, Morgan (Lexi Rabe); her cameo would have occurred right after that climactic snap of Tony’s fingers. “There was an idea that we had that Tony was gonna go into the metaphysical way station that Thanos goes into when he snapped his fingers,” Joe Russo explained
The scene was shot but didn't seems to work as intended and was removed from the final cut. I don't know about any scene with Bruce Banner.
"We showed it to a test audience, and it was really confusing for them." (...) Joe also clarified further: “The intention was that his future daughter. (...) And the idea felt resonant. But it was just too many ideas in an overly complicated movie.” “What we realized about it was we didn’t feel an emotional association with the adult version of his daughter,” Anthony Russo added. “It wasn’t ringing to us and resonating with us on an emotional level, which is why we moved away from it.”
In-universe, we don't know how the metaphysical realm is working. It seems that it is related to the Soul Stone because of the colours of the scene with Thanos and Gamora but we don't know whether it is an automatic consequence of the use of the Soul Stone (the stone wanting its holder to reflect about his/her actions?)
My guess is that, contrary to other stones like the Power and Time Stones that seem to be powerful passive tools, the Soul Stone is sentient, at least to some extent: it can understand the soul of its holder and his/her intentions. This solves some questions related to the stone:
If this is correct, then it follows that the user of the Soul Stone is not automatically transported into the metaphysical realm. Thanos went there either because he wished it or because the stone felt he needed it. Clearly, talking to Gamora gives closure to his quest. Probably, Hulk and/or Iron Man didn't wish/need that.
Note that, it is not because we didn't see such a scene that it didn't happen. Maybe Tony visited the Metaphysical realm to talk with old Morgan. Maybe Hulk visited the Metaphysical Realm and met Black Widow there.
Edit: I see two problems with my theory:
why are the Avengers allowed to use the Soul Stone twice? After Hulk snap half of the Universe back into existence, what Black Widow sacrificed for has been realized, so the Soul Stone should have required another sacrifice. I guess this is solved by the fact that Tony knew he would die for sure, hence sacrificing himself to the Soul Stone (nothing really says that the sacrifice has to be made on Vormir).
The biggest problem: why would Thanos be able to use the Soul Stone in the finale battle of Endgame? He is not yet the Thanos that sacrificed Gamora, and Black Widow's sacrifice means nothing to him.