I guess the best answer is: "Because anyone is doing it this way."

I suppose AR is defined as it is because most consider the performance of a system to be profit per risked capital. The capital used for fulfilling broker requirements, such as margin, is not under risk and thus unrelated to a system's performance.

With most strategies the drawdown is anyway much higher than the margin. Exception are systems that heavily hedge. They have low risk, but need much capital for the margin. As this capital is needed, you're theoretically right that it should affect the performance. But the AR is just not defined this way.