Just because someone's an Avenger doesn't mean they are capable of handling every situation; especially a situation like this.
Big baddie that is attacking the world? Yes, call the Avengers. The purpose is to kick ass and win the day.
A super powerful ally who has created an alternate reality bubble? What the hell is Cap, Antman, Hulk, Cpt Marvel, Spiderman, Thor, or most any hero going to do about this other than just stand in the command center and wait for someone to figure out how to get in there?
They've already tried to communicate with Wanda, and that failed. Bringing in an Avenger to communicate with her seems useless.
The only hero that makes sense to bring in is Dr. Strange. This kind of stuff is right up his ally. The question is, what level of knowledge does SWORD or any agency have of Strange and his crew? That is something I do not know.
I disagree with a previous poster about every event being something the Avengers should handle. Each movie has addressed that when it happens. Ant Man literally mentioned calling the Avengers, and Pym basically shrugged them off and said they were busy dropping cities on other cities, or something like that. Peter asks Iron Man about bringing in the Avengers in Homecoming, and he said that was below their paygrade and called the FBI. In Far From Home, Fury literally references each hero when Spiderman asks.
When you have to do work around the house, wouldn't it be easier if all your friends came over to help with everything? Well, yeah. But you can handle most things yourself and they have lives of their own with their own problems. When you have to move a fridge up to the second floor, maybe call a friend or two to help.
This expectation that every Avenger is just sitting around to come help with any little thing that happens is sillier than "the Fellowship should have just ridden the Eagles to Mordor." At least that argument is in regards to a possible world ending situation.