The sort in standard library should have predictable performance, so "usually" is not good enough. Moreover, to do random pivot, you need to have a good source

The sort in standard library should have predictable performance, so "usually" is not good enough. Moreover, to do random pivot, you need to have a good source of randomness, and if you don't, an attacker often could predict the sequence of pivot choices, and prepare the input to ensure "random" choice of pivot results in a quadratic performance.