Of course I can’t look inside of the developers heads, but I will try anyway to hypothesize how we got to this situation.
Probably the developers were thinking: ‘If we use the average rating in our calculations, that can be really unfair when e.g. a 2k player and a 1k player match up against two 1.5k players.’ So they decided to let rating gain/loss be dependent on how your rating compares to the highest rated player on the other team. Now in actuality, they may not realize it but their solution did literally nothing to solve the issue it was intended to solve, as the actual MATCHMAKING system still looks at average Elo. The only thing the solution did was introduce an additional problem: Rating inflation.
Developers have the tendency to want every change to be a strictly positive change, and to never go backwards. They may be thinking: 'Yes, @WoodsierCorn696 s solution would solve the problem of rating inflation, but it does not address the initial reason why we have the current system, and so his solution may worsen the problem of people of wildly different rating teaming up (even though they would be incorrect in thinking so).
I bet the developers only want to change the current system if they come up with a new system they are fully satisfied with, and they cannot afford the time of thinking about this problem for too long.
By the way, I don’t even know if the Elo inflation itself is the biggest cause of unbalanced team games. The current system has a load of other side effects that seriously imbalance team games, such as experienced by the OP of this post: Team elo is not working
I myself have been the unwilling cause of imbalanced teamgames… I have played lots of 4v4s with three friends. When we started playing together, our team Elos were quite different, but after a while they became almost the same (the high rated players in our team would get few points for winning, and the low rated players many, and vice versa every time we would lose). Last weekend, I played seven 2v2s with one friend, and we are the strongest members of the 4v4 group. Because our rating undervalued our skill, our matchups were too easy, and we won all seven of our games… Previously I wrote that this current system probably was designed to mitigate the problem of people of different ratings teaming up, and I said it did nothing to solve the issue, but that was an understatement: it is making it worse.