Shouldn't you like start in bronze 5

No. Bronze 5 is the bottom of the barrel in terms of play quality, the left side of the bell curve. The vast majority of people are at the average skill level, Bronze 3 - Silver 3

Shouldn't you start in bronze 5?

All new players or NEW ACCOUNT start at Gold MMR and then work their way up or down depending on how they do in their placement match this Season before it suppose to change in 2019

That's why Gold Players and Previous Plat Players from last season are fuck because of this system, if you're a low Plat or a Plat players with low MMR, you'll end up with new possible players in your game and get fuck especially if they are on your team. And it is also possible that you will be fuck as well if you have actual Gold player that weren't even Plat on your team against actual Plat players who got place in a gold MMR match.

So that's why the Rank system and MMR is shit because of this. You don't know what kind of Team mate you will get and what kind of players you will play against, its RNG, you win depending if you have the better players