Why am i still playing with silvers when im in my gold 3 promos?

So out of the last 20 games, I have a 75 percent win rate, and I'm still playing with silvers (Im gold 4 in my promos to gold 3). Is it impossible for my mmr to increase right now? I still end up playing with people who know less about the game than me, for instance, our fed vayne builds pd and ie instead of stattik shiv and ie where shiv would help us waveclear and stall out the game. Our cassiopeia also can't stay under tower and keeps stepping out to cs where yasuo just goes in on her and takes 75% of her hp down. True, I played poorly that game, but for once, why cant one of my teamates carry instead of me? If you check my match history, I have a 75% win ratio over the last 20 games. Why am I still playing with such low players?