What is the point of position ranks if I'm just going to be matched based on my highest rank anyway?

Title.

I play on my university's Legaue of Legends team as the support, so I spend a lot of time practicing by duo queuing with our ADC. I'm currently Platinum 2 as support, and he's Diamond 4 as ADC. Played a game where I got filled to my secondary role, mid, where I am currently still in placements and will probably land around Gold 2 when placements are done.

I have very little confidence in my ability to play mid lane at the level my support games have been (around Platinum 1-2 mmr). The game of course was matched as if I had gotten support, with the average ranks around Platinum 2.

Played Orianna into Yasuo, so I pretty much just sat tight and played safe all lane and tried to soak up as much xp and cs as possible while not dying so I could be relevant in team fights later. We ended up winning that game but why do I have to play against players of higher elo on their main role when I'm on my off role?