I understand that it is possible to cool a liquid past its freezing point, and still maintain its liquid state because crystallisation would not happen in the absence of a seed crystal.
My question, was marked as a duplicate and I was linked to to Can all substances be supercooled?
One of the answers over there does address my question but only in part. Here's what it says:
"However, when there is only liquid present, you will first have to form a small crystal nucleus which is not as stable as the bulk solid i.e. has a higher free energy. This is basically a kinetic barrier which prevents forming the solid. The formation energy of such a nucleus will depend on the interfacial energy of the crystal-liquid system. In a substance where this interfacial free energy is very little there won't be too much supercooling."
My question still remains. Why is the lack of a nucleus a kinetic barrier to the formation of a bulk solid? If you continue past the glass transition temperature, we form an amorphous solid anyway. How do the mechanics of vitrification and crystallisation differ in this regard?