DC output of differential amplifier with active load
Because it's a differential amplifier.

M5 provide the bias current \$I_{Q}\$ for the differential pair formed by M1 and M2. When a common-mode voltage of \$v_{cm}\$ applied to M1 and M2's gate, the current \$I_{Q}\$ splits evenly between M1 and M2. Just this current biases the transistors.