Yep, this is probably one of the number one assumptions you have to make to keep yourself sane while watching these shows and movies. You just have to assume there is always some good reason that the other superheroes don't come to join in.....because if this was "realistic", of course all of the superheroes would join in on these global level threats, it would be silly that they wouldn't.Good points all, but even if all the Avengers were alive and waiting by the phone . . . .
This is a common trope in superhero literature, why doesn't the Flash just call Superman when things get tough? Or just group-text the Justice League?
The simple answer is, this story is about the Falcon and the Winter Soldier. It isn't about the Avengers. Rhoadey made a cameo in episode one to help ground the series in the larger narrative, but this isn't "The Falcon, the Winter Soldier, and Warmachine".
If your ability to suspend disbelief ends when Sam and Bucky don't call in whatever remaining Avengers to save the day . . . . perhaps superhero shows are not the genre for you.
So everytime the question in your head whispers "but why isn't XYZ not here", you just have to answer "because of ABC reason that's not spoken on camera". Its the requirement of disbelief to enjoy the genre.