We do it that way to keep things simple. Selling stock by the dollar can be a little tricky because you won’t know how much stock you’ve sold until the end of the day when the trade executes.
Let’s say you own 0.9 shares of Apple stock worth $100 and want to sell all of it. If you placed an order to sell $100 of Apple, and the price of Apple stock goes up during the day, it will be worth more than $100 when the order executes, leaving you with a small amount of Apple stock after execution. That is probably not what you wanted.
On the other hand, if you place an order to sell 0.9 shares, you will sell all 0.9 shares. If the price of Apple went up during the day, you’ll just receive a little more cash from the sale.