These are not additive however. If you are a daytrader for instance you will buy and sell, say amazon, multiple times in a week, and will clear your portfolio

These are not additive however. If you are a daytrader for instance you will buy and sell, say amazon, multiple times in a week, and will clear your portfolio over the weekend or holidays etc. If you do statistical arbitrage/mispricing you keep for weeks usually and you have to account for slippage. (You always have to account for slippage.) So a 0.5% trading back and forth trying to converge on a price and meet some new demand either way is stable state and it can involve the same players (buyers/sellers incl. market makers). Now imagine if the market suddenly demands an extra 5-10 billion of dollars per week on top of the existing stable demand. The money has to be introduced in the market somehow. It means that for me it must be worth it more to buy amazon over something else in my investing horizon. And even with margin I expect this is going to raise the collateral the clearing houses would require. Now also imagine a SEC entry (form 4) stating that 4% is dropping on the market. A recent example is Elon Musk with Tesla and he tricked people into thinking it was their choice and to my knowledge he still has not sold that whole 10% of his position.(Not 10% of the company...)