Placements

So im a little confused as to why exactly the placements for ranked are designed for you to fail horribly. i was 5 and 0, op.gg rated me saying id be around gold 3 with 1500 mmr. i lost A SINGLE GAME and lost 475 mmr and now im likely bronze 3. What the actual hell is that???? if you arnt literally perfect in every game you play in placements you drop full divisions with a single loss now. This is the exact reason why so many people dont want to play ranked. the system is horribly broken