Glaciers do not form simply because of cold. In order to have glaciers, you need to have at least two things. You need to have a great deal of snow falling in the winters. In addition, you need the climate to be cold enough that the snow does not melt, but rather compacts into ice and forms a glacier.
So, if Siberia has the cold but has no glaciers, then it must be missing the other part of the equation. Siberia does not really get enough snow to make many glaciers.