Right, that's why no one enjoyed the movie "Sneakers". Also that's why no one would ever do a movie about seven samurai. 7 characters, and they're all samurai? How boring! How could you even tell them apart?
That's why each of the "Ghostbusters" had unique abilities. Imagine how boring "Ghostbusters" would have been, if all four of them were equipping identical proton packs, firing identical streams of energy at ghosts.
That's also why no one watches soccer teams or basketball teams or hockey teams with multiple players in overlapping roles, and that's why there are no famous rock and roll bands from Liverpool with two guitarists. (Or, worse, two guitarists who are ALSO both vocalists. Imagine how badly would they step on each other's toes; it's easy if you try.)
In short, all of these would be epic fails, because people only have solo abilities, and don't have interesting personalities or motives or methods which are independent of those abilities, and cannot combine their efforts in interesting ways.
I have not seen Sneakers in a long time and I have never seen 7 Samurais but in Ghostbusters they may have worn the same equipment but they had different roles. Veckman was the face, another was the expert on science and another on lore so they had things that made them different and stand out. Which is something I believe 5E is trying to do with customizing your PC. So you could have two wizards of different schools who are not stepping on each other toes you could have a group of fighters who specialize in different fighting styles.
As for you example about sports well in football there is only one quarterback on the field at one time with bands both are not singing lead at the same time. Also with sports some players don't play in a game they set on a bench I don't think any of are suggesting that would be a good way to play DnD.
But if you look at most TV shows most have a format with people having different skill sets take Star Trek there is only one captain, one first officer, one head engineer, one head doctor yes someone may fill in for the captain or Data may do Geordie role if he is off ship or doing something else.