OK, in what sense is the market fixed?
The purpose of the stock market is to allow companies to raise capital by allowing investors to share in their profits. I can buy 5.4 x10-9 of the profits of Alcoa (just an example) for $18.70, and this will get me about 12 cents per year. For this to work, there has to be a secondary market; nobody will buy a share of stock if they need to hold it for the rest of their lives. "The market" is really driven by the secondary market. It looks like Alcoa last issued shares at the end of 2019. What if I want to buy stock today? And what if someone who has it doesn't want it anymore? That's what the stock market tries to do. I would argue that it performs this function reasonably well.
Company prioces change over time. Thus, the value of your five-billonths of Alcoa is worth more on some days and less on others. There is money to be made speculating on which way this will move. Even if everything is above-board, the individual investor is at a serious disadvantage here. I can spend nights and weekends studying Alcoa, but JPMorgan can have a person whose entire job is to follow Alcoa. Next to his desk is someone who studies only Kaiser Aluminum, and they have a boss who supervises the entire sluminum market and so on.
Big banks will do better at speculation than individual investors even without skulduggery because they are better at it. They are better at it because they put more into it. Individual investors better understand that. Ignorance of the facts is no excuse. Of course, if one plans on holding stocks for a very long time, this doesn't matter so much. The longer you hold it, the less it matters.
It is certainly true that brokerages make their money in ways that are opaque and in some cases legal but unsavory, e.g. profiting off the bid-ask spread off their own clients. But it is also true that trading has gottem much cheaper. Fifty years ago, a guy named Charles Schwab made his name offering trades for a mere $70 commission. Back then, you paid extra for odd lots (not a multiple of 100 shares), extra if you were a small investor (or, if you like, a discount if you were a large investor), had a bid-ask spread twice what it is today, didn't get your trade executed in seconds, and so on. Would I prefer a more transparent model, without the monkey business? Absolutely. Do I want to go back to 1974? Not on your life.