Why Exchanges do not allow to handle orders individually instead of using average price?

As the title suggest, i wanted to understand what are the differences between managing the trading positions/orders individually rather than using the average price for buy and sell. There could be some typo so feel free to point it out to me if you notice it.

===== Example 1: Exchange give the user the ability to handle trade position individually

Operation 1: BUY 5 ADA at 0,50 usd

Operation 2: BUY 5 ADA at 1,00 usd

Operation 3: BUY 5 ADA at 1,50 usd

- Close Operation 2 and 3

SELL 5 ADA at 2,00 usd = 10 usd

BOUGTH 5 ADA at 1,00 = 5 usd

Gains = 5 usd

total capital for this position = 10 usd

SELL 5 ADA at 2,00 = 10 usd

BOUGHT 5 ADA at 1,50 = 7,5 usd

Gains = 2,5 usd

total capital for this position = 10 usd

- Leave Operation 1 open

SELL 5 ADA * 2,00 = 10 usd

BOUGHT 5 ADA * 0,5 = 2,5 usd

Gains: 7,5 usd

potential total gain = 7,5 + 7,5 = 15 usd

======= Example 2: Exchanges give user the ability to handle position based on average price

Operation 1: BUY 5 ADA at 0,50 usd

Operation 2: BUY 5 ADA at 1,00 usd

Operation 3: BUY 5 ADA at 1,50 usd

- Close Operations for 10 ADA based on average price

Average Price: 1,5 usd

SELL 5 + 5 ADA at 2,0 = 20 usd

BOUGHT 5 + 5 ADA at 1,5 = 15 usd

Gains = 5 usd

- Don't sell last 5 ADA but calculate its potential gain

SELL 5 ADA * 2,00 = 10 usd

BOUGHT 5 ADA * 1,5 = 7,5 usd

Gains: 2,5 usd

potential total gain = 5 + 2,5 = 7,5 usd

- Conclusion

From what i have calculated (if i didn't do any mistake), we have a potential gain of 15 usd if the exchange offered us the opportunity to manage every single operation separately and a potential gain of 7,5 usd when we handle the operation with average price as all the exchange are currently doing.

Why there is no Exchanhe platform that offer us the ability to manage our positions individually?