Let's give an example of you bought 100 BTC at 9000$ Price and the price went 9010$ in seconds then you sold, so your profit will be 1000$, may 500$ fees, but the another 500$ you made it profit in seconds!
Let's discuss at the point!
You are correct some points if we are not going to consider the fees here your calculations are correct you could really earn that amount easily. Buying low and selling it for the high, it's still the best method.
It is just how the market works the higher the volume you buy for sure when the market price of bitcoin spikes the higher the profit you are going to get, but it was a big investment since you are buying a high volume of bitcoin also investing in high volume includes a big risk the easier you could earn a big profit the riskier its gets since when the market price drop even for a low amount you could easily lose a big amount of money.