Why is teaming frowned upon?

I mean for the most part I never team in solo battle royals, but I found myself caught up with a friendly after assisting each other with a quest. So it made me wonder if you do not even know who you’re teaming up with in a match what’s the problem with it, it’s not like you not going to have to kill each other anyway. Why are people so upset by 3rd parties also, is that not similar yet it goes by the way side often as well.