Sometimes it is done to not let the MC get overly powerful or rich.
In reality, the butterfly effect should impact the stock exchange heavily a decade after the MC plays with the market.
Except for some specific events, you wouldn't be able to rely on your future knowledge after you played the market for some time.
The money you make, won't be available somewhere else, you would invest differently then someone had originally done.
Let's say you brought stock from a company that you knew was going to skyrocket. Some other broker would have done the same in the original timeline, but now the stock is pricier because you bought x amount of stock and the other broker had a cut-off price that was going over the amount he decided was worth it.
So all the money he would have invested was going into another stock.
Second scenario: In instances where the stock price wasn't directly influenced by the release of a product or the events in the world, you would have a stock that was going up steadily just on loyalty and long-term-planning. Again, if there were one or several other broker who had a cut-off price, you would basically direct their money elsewhere, and what if it was Warren Buffet or some other influential broker who uplifts the stock just because he has trust in the company? Or a big Investor who takes on an influential role inside the company either because he threatens the company with his stocks to do xyz or is given a seat on the board and is a very positive influence on the direction of the company?
Sometimes investors have to threaten with a sellout to get the company to change things, maybe you as a big investor had to steer the company in a specific direction to get the stock price you remembered, but this would not have been public knowledge.
Third Scenario: You would dump all your stock on the highest price you remember because you know it wouldn't get higher and it would have been stupid to hold onto it longer. This would negatively impact this company and the price would be lower than in reality, possibly upsetting a whole bunch of things. On the other hand, you can't even know the highest point of a stock because you changed it with your own investment.