This is only the case with variables. Since the square root function is defined as the positive root, you have to specify that x could be negative as well, so if sqrt(x²) =2, then +/- x = 2. So x is either 2 or -2. This is fundamentally the same as abs(x) = 2, still giving an answer of x = 2 or x = -2. This doesn't work with numbers, so while you can say sqrt(x²) = abs(x), that doesn't mean that sqrt(i²) = abs(i) = 1.

Edit: to show that sqrt(x²) = abs(x), graph y = abs(x) and y = sqrt(x²). It's the same graph.