This literally makes no sense

what were you last season

You were Bronze 2 at the end of the season.

Players generally drop around a tier after placements.

Bronze 5 was the lowest you could drop.

It's based on where you ended last season and your new MMR. When the reset happens, MMR becomes (last season's MMR) + 1200 / 2, which is why most people drop after new placements. Typically, it's not easy to get your MMR up to above where you were last season, since it's got 1200 (the average MMR for all of League, about equal to silver 5) added to it and then divided by 2.

Also, platinum 1 is the highest you can get placed. Imagine being challenger and having to drop back down to plat 1 every year and then having to climb back to challenger afterwards. The reason you don't see them complaining is because they're confident that they can climb back to where they ended the previous season with relative ease. Try getting out of bronze if you don't want to get placed in bronze. Maybe you should've done your Flex placements first, since your Flex Queue rank will affect your solo queue rank if it's better. If you truly deserved to be out of bronze now, you would get placed higher than that in Flex because it's a hard reset, not a soft reset.