Because the "vector networks" and vector graphics editing are for the graphic de...

Because the "vector networks" and vector graphics editing are for the graphic design part of Figma, not the UI design one.

You don't just draw whatever random vector shape and turn it into UI.

You create UI elements and arrange them in UI related containers (grids etc) for that.

The "vector drawing" is for the decoration of the UI, not for its definition.