The constitution of the US (a liberal system) contains this phrase you might be familiar with. "All men are created equal". Equality is a central tenet of liberalism.

The argument that your parents make you not equal is exactly what liberalism is trying to reject. Someone should not be treated differently based on their heritage (monarchies anyone?).

In fact, since you believe you can't be equal due to parentage, you probably agree with some of what progressives have to say. The current system isn't equal.