> Traffic can only approach you from one direction
But from two axes in that direction; the entry-lane immediately to the right and then those already in circulation on the roundabout. And often two or more lanes on each of those axes.
When those in the nearest entry lane hold-back, something is approaching in circulation. Change focus to determine its lane, destination and rate of approach and if there is time to enter ahead of it.
For many modern roundabouts the view across the circle is deliberately obscured by vegetation for some reason.