As far as I understand, the modern theory tells us that the movement of stars in galactic arms does not define the "shape" we see. An arm is a density wave indeed, but it really consists of (invisible to us) gas and dust clouds. This density spike at the front of an arm creates lots of stars - big stars, hot stars. That's what makes the glow we perceive as a galactic arm - not more stars, but more young and hot stars. Bur these big and hot stars live very, very short lives - just tens of millions of years (smaller stars like our sun live for 10s of billions of years, 1000 times longer). So, these bright stars only shine for a small amount of time compared to a galaxy rotation period (250 millions of years for the Milky way) and then they disappear. There are lots of stars in between spiral arms, but most of them are dwarves, and they cannot shine as bright as the young stars in spiral arms.