There is a practical difficulty to everybody being liberated at the same time. Different people are at different stages of evolution of their own spiritual consciousnesses and the resolution of their own desires, with respect to the four so-called legitimate pursuits of human life, known as purushartha in Sanskrit – पुरुषार्थ.
These four pursuits are:
धर्म – dharma – duty, righteousness and moral actions
अर्थ – artha – wealth or more accurately, material wealth
काम – kama – pleasures of the senses
मोक्ष – moksha – liberation from the cycle of birth, ageing, disease and death
There is no way that billions of human beings, and other beings, are going to be on the same page or stage where they realise that the end goal of mortal life is the fourth and last पुरुषार्थ – धर्म.
Assume for a moment that by some quirk of fate, or miracle, everybody was indeed on the same page and ready for liberation.
The challenge we face is that if everybody was liberated at the same time, the material world as we know it, would have to cease to exist.
However, that cannot happen as it creates a paradox. The paradox is that the material world cannot cease to exist. The reaosn being, it is the platform on which we get to play out our Karma.
We have to play out our Karma, till we have exhausted them, or have earned positive Karma to completely negate our negative Karma.