I agree with you that challenges shouldn't make you sabotage your teammates, but designers are generally looking for interesting things for players to do, which is why you see this pattern across many games. There's also a deeper issue around the player experience that affects this.

When developers playtest, they're doing so with each other--in their private environment, there is no solo queue. Thus, they tend to design experiences that work in a situation where everyone knows each other, is communicating, and is understanding of the mutual goal--in other words, playtesting is almost never indicative of the real player experience in a multiplayer game. This has been the case in every studio I've worked at, and has resulted in problems that trickle down to the player. It's nigh unsolvable.