The "problem", or rather the issue (because it is not a problem), is that you were essentially already placed BEFORE your placement matches.
Here is a rough sketch of how it works.
Your MMR, which is a hidden value, goes up and down based on who you play.
When you play, the game attempts to place you with individuals with an equivalent MMR.
It may very well be that normals are much more relaxed on game composition, whereas ranked is very strict.
Anyways, the game attempts to create a pool of 10 players of roughly equivalent MMR.
But this system is not perfect, sometimes a team will have a combined MMR that is higher than the opposing team.
Either way, it is irrelevant, because MMR is essentially a "bet" on the odds that you will win or lose playing against a spectrum of lower, equal or higher MMR players.
MMR, NOT LP, determines "what level you are on".
When you begin your placement matches, the system takes your current MMR, and PLACES you against opponents that are at that MMR.
You may move up or down from that point, but the gains and losses are largely irrelevant, because you are playing people that have the same MMR as you.
You CANNOT jump to gold by playing bronze players, and if your MMR is bronze, well, there you go.
Your MMR probably can and does shift during placements, but not by large margins.
It would probably be better if, during placements, the system actually scaled you faster based on wins or losses.
Win a game in bronze, go up to silver, win there? Go to gold. Lost there? Go back to silver.
Over 10 games, dial you back in to where you need to be.
There are many problems with doing it this way though.
First, it would inject a relative unknown element into different competitive levels of play.
That is VERY bad, as while it would be testing your skills, it would also be skewing live games composed of people with established MMR's.
Finally, here is why victories and losses during placements don't really matter.
If you win against teams that are lower than your MMR, well obviously you don't go up much.
If you lose against teams that are higher MMR, again, you don't go down much.
In order for placement matches to really shake up where you are and where you are going, it MUST place you outside what it feels is your "comfort zone".
Thus, if you take a plat player, plug them into a bronze placement series, guess what? They will still probably place bronze.
The LP system is not broken, beyond the asinine requirement of promotional matches, which serve no actual purpose.
What MAY be broken is the MMR system.
I DO know there are plenty of ways to completely tank your MMR.
And the lower your MMR going into a placement series, the harder it is going to be to dig yourself out.
At very low levels, you are playing with people that have no clue how the game works.
They are essentially playing a different game with different rules than you.
That is why they are at the bottom levels.
They are also fundamentally retarded(literal meaning, not derogatory) from improving, for whatever reason.
This could be intellectual capacity, interest and intensity of competing, psychological inability to recognize personal responsibility or an inability to conform to team standards.
Again, this is why they are at the bottom levels.
This is ELO hell, a place where you are constantly playing in the Special Olympics.
While this may seem mean, I don't intend it to be. Generally, your team is handicapped on fully realizing the game mechanics. Unlike the Special Olympics, MMR doesn't let everyone be a winner and everyone does not get a gold star at the end. MMR treats a brand new player the exact same way as a Pro player.
So think about that for a second, because the implication is really screwed up.
The MMR system does not recognize individual effort, the only metric that matters is a win or a loss. If you played well, but your team did horrible, MMR will rank you at the same level as the rest of your team. This would work well if occasionally you were carried by high MMR teams, to offset the times when you lose because of a low MMR team. But it does not work that way.
Starting at about silver 4 and up, people generally care about winning, and at least have some fundamental grasp on how to do it.
BELOW that though, you are playing with a community of players that is LARGELY inept at playing the game.
Some of this is lack of experience, some of it is due to an inability to learn or a low motivation to compete.
I would very much argue that placing ANYONE in the lowest tier of the system will greatly inhibit their ability to reach their actual potential, because they MUST exhibit far superior skills to make up for the ineptitude of their team.
A win in bronze should mean ALOT more than a win in any other division, but I suspect this is not the case.
Anyways, I have wandered far off topic.
The point is, unless you actually play AND BEAT a diamond level team, you don't even get a chance to compare your skill level with that MMR. And placements won't test you there.
Placements will test you at the MMR it assigns you AT THE BEGINNING of the series.
From there, the series is a drop in the bucket, 10 games up or down on the MMR scale.
And it is pretty much impossible for your MMR to dramatically change in 10 games.
It is designed NOT to do that.