It's really not.
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Since spoiler protection has been requested, I will quietly point out that TFaTWS also gives some explanation as to why Bucky and Sam, specifically, were not there to support Wanda. They had their own severe problems to deal with. We are told that Sam tried to text Bucky and got no response. It’s plausible that Sam also shot some texts Wanda’s way and got ignored. If we want, we can also fold Wanda’s situation into Rhodey saying everything is coming apart and some friends are now enemies.
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I just think of this in the way I've always thought of comics. Some characters just never are gonna be in there no matter how much it'd make sense for them to address situations. They have their own stuff going on. And it's not like Falcon and Bucky were close to Wanda. They stood beside each other once. I don't want Wanda to be an enemy, so I hope that doesn't get folded into her. I want them to grow the character. And she can't grow if she doesn't get a hold of her powers and learn from them. Which is what she seems dedicated to doing.
Love is for souls, not bodies.
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Oh, sure. In actuality, it is all driven by practical necessities of storytelling. But the situation in-universe, so to speak, is not as inexplicable on its own terms as some make it out to be. As for the second point, I think we will see a move toward antagonist rather than enemy, but it’s easy to see why it might be phrased that way.
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I don't even want antagonist. They already had her do that with Ultron. I want to see character growth. Wanda doesn't deserve that complete assassination of her character.
Love is for souls, not bodies.
I just extracted this part since it's not a spoiler for anything, but Falcon and Wanda were on the same Avengers 2.0 team. I don't know how much time passed between the end of "Age of Ultron" and "Civil War," but they certainly trained together and lived at the compound together. Then they were all in hiding together after "Civil War" and that was .... two years? I doubt they had any lines to each other, but they certainly spent enough time together that they should have been fairly close. But I know it doesn't fit the narrative of Wanda not having anyone.
Okay, the spoiler stuff is getting cumbersome so sorry if this reveals too much. We have to remember that Wandavision was three weeks after Endgame, when everyone was floundering in a sea of their own troubles (Sam with his family finances, Clint with his loved ones and his problematic actions, Bucky with his everything) and we do have evidence that Sam, at least, was trying to keep in touch with people. Wanda does not seem one to accept offers of emotional aid except from someone she was very close to, and all those people are, unfortunately, gone. She has already obviously accepted practical help from someone (the car, tracking down Vision's body, probably regularization of her status) and likely that is already making her feel awkward.
Like has been said, all of this obviously is due to the practical necessities of storytelling (this is comic book adventure, not soap opera, after all). Nevertheless, I have experience of trying to help a stubborn, hurting person (I should say I have been on both sides of that situation), and connections or not, I don't think it's all that unbelievable or incredible that Wanda should be in the situation she's in, however tragic that is.
Also, Wanda is beginning to rebuild connections, first with Monica Rambeau and then, hopefully, with others. That also is not unrealistic for how people come out of isolation and mourning.
I would say, with a few exceptions like IM3, Endgame and possibly a couple of more, most movie events happened in the year they were released in.
Sam probably prioritized his family. Bucky prioritized on his mental health and the guilt of his actions as the WS. The Avengers are like co-workers in an organization and not everybody is going to be so close that they will drop in on each other's personal lives. The Falcon and WS show already makes a point that the only common thing Sam and Bucky had was Steve and they couldn't otherwise stand each other. Maybe, Wanda and Sam weren't particularly close but they were both loyal to Steve as he was the glue that held everything together.
I wonder if Black Widow will flesh that out a bit more - the time they spent after Civil War. Natasha goes her own way at some point (if the timeline of the movie is what they're telling us) and from what I recall, Wanda and Vision had to "sneak away" for brief periods since he was on the other side. I imagined she was with the others when she wasn't with Vision, but that was an assumption on my part.(especially since the others were so close when W&V were attacked) I guess we don't really know how she spent those two years aside from the stolen moments with Vizh.