Because people are dumb as fuck and think that duoing automatically means you got carried. Is that the case sometimes? Definitely! But not always. I dealt with that mentality for so long while climbing of bronze, finally hitting gold and getting shit for it. Once I hit diamond people stopped bugging me and now they ask to duo instead. In the end the way I see it is, if you got carried and want to boost your ego and lie to yourself that you're good, you do that bud. I've played with a friend which I think doesn't even deserve to be gold 4 but I play with him to calm him down and he always manages to demote/lose all the lp I helped him get by duoing. Everyone else I play with maintains their rank or climbs higher. The first friend is someone who would probably put you down for climbing with a duo, because they themselves can't climb.