Because this game uses the MMR system. Currently, your MMR is in gold 3, and all of the Plat players you're matched with have equal or lower MMR. In other words, the game considers both you and these Plat players to be gold 3.
In theory, this system is perfectly sound, as once your MMR gets to where it should be, your winrate will stabilize, because you're playing with and against people of similar skill, but the disparity between your LP gains and losses will cause you to climb divisions until your MMR and rank match. It's also more fair to actual Silver 1 players who are significantly worse than you.
Of course, that's not what happens, because Riot has these things called "promo series", which make the MMR system work awfully, because it makes climbing take years longer than it should.