let me explain why they don't do a hard reset. Lets say it was a hard reset and you went 10-0 in placements and lets say the highest place you could place was still plat 1. Do you know how many bronze players who got lucky and 10-0'd would be placed with challenger and master players. Then challenger players wouldn't get competition and bronze players would feel the competition is to steep. Then you could get boosted to even higher ranks.
The season before this i ended the season silver 2 got placed in bronze 1, climbed out of bronze 1 in 1 day. This season i finished gold 5 got placed in silver 2. As you can see im slowly but surely climbing. If it wasn't based off previous seasons every year i could potentially start from bronze even though i know im atleast a plat player.
And if you get placed in bronze that means you just gotta climb and trust me once you reach my level bronze really isnt that hard just gotta wanna improve.