Gain vs Output Impedence
In a single amplifier stage you can't have low output impedance and high gain at the same time. The active device (bipolar or MOSFET) is voltage controlled and generates a current. In order to get a voltage again you need a resistor. Therefore you have to make a tradeoff between gain and low output resistance.

The output resistance is only of concern when resistive loads are driven. In analog IC design the loads are often capacitive and OTAs (amplifiers with a high output resistance) are perfectly fine for such a task.

If it is needed to drive a resistor either a buffer stage (emitter/source follower) or feedback is required to achieve that goal, which usually requires a multi-stage design.