Why do we use one bias (VCC) for transistor amplifier circuits and 2 supplies for opamp circuits?
A supply rail and a biasing point are different things.

Some opamp circuits use a negative and a positive supply rail because sometimes that makes the circuit simpler. 

Here are four basic opamp circuit, the inverting and non-inverting amplifier for both single and dual supply configurations:


Source

Note how extra resistors and capacitors are needed in the "Single supply" circuits. The resistors are needed to create a 6 V DC voltage (to bias the opamp properly) and the capacitors are needed to block that same 6 V from reaching the input and output.

Using a negative and positive supply rails can also be done with transistor circuits. Many audio amplifiers do this, for example, this circuit contains no opamps but uses a +40 V and - 40 V supply:



Source

Basically this circuit is a "power opamp" with feedback.

A biasing point is the current (or voltage) at which we bias a transistor. It is an operating point, for example we make 10 mA flow through a transistor and allow the signal to make that current vary between 5 mA and 15 mA.

This can be done with a single (usually positive) supply but also with a negative and positive supply. Which one is used is irrelevant to the biasing of the transistor.