The placement matches automatically lower you at least 1 full ELO even if you win all of them, the point being how the tiers work. At any time in league theres a general % of players in every ELO. Notice how as you go up, you generally gain less LP for a victory and lose more for a loss. It's something like 25% bronze, 50% silver, 15% gold, 8% plat, 1.8% diamond, .2% challenger (guesstimated)
In bronze you gain 20-30 LP for a victory and lose 10-15 on a loss making is easier to get out of bronze
In silver you generally gain 10-20 LP for a victory and lose 10-20 LP for a loss. Silver is where pretty much all the average players are placed, and an average win/loss rate (50/50) will keep you there.
Etc etc the higher you go, the higher win rate you need to move up.
The better players move up and push the bad players down.
It was especially weird at the very beginning of the season when you would commonly see previous plat/diamond people placed into bronze and silver despite winning like 9 games or something, the farther along the season goes, the more lax placement matches become.