I play mostly team games and I’ve noticed that when your team wins the higher ranked teammates gain fewer points than the lower ones, and that when your team loses the lower ranked players lose fewer points. The gain/loss also depends on the other team of course, but the important point is that the algorithm compares your individual scores with the other team’s scores.
Here is the problem: Suppose you have one strong player teaming with one weak player. The algorithm is probably going to give them opponents with an average skill level somewhere in the middle. If the matchmaking is roughly fair*, they should win roughly half the time. But the strong player is going to be losing a lot more points each time (s)he loses than she gains when the pair wins, and the weak player is going to gain a lot more points with each victory than (s)he loses when they lose.
If they play multiple games their elo is going to converge, and the stronger player will effectively be giving away elo to the weaker player. This will of course cause mismatches in skill level if they ever decide to play with other teammates or enter the queue alone.
*You don’t actually have to assume fair matchmaking for this argument to work. Just note that the expected elo change is p(win) * gain - p(loss) * loss. Playing together means you have the same p(win) and p(loss), but the higher skill player will have a smaller gain and a larger loss, so there is no way both of these expectations can be zero.