The rocks are reminants of volcanic eruptions exposed by erosion. Oregon and Washington have many volcanoes the same as you will find on the Pacific Rim.
I suspect the east coast had the rocks removed while Oregon values keeping the beaches natural
Age ..... west coast is younger then the east coast. The tetonic plates are hitting each other head on.
The Pacific Coast is much rockier. Sandy beaches are there just separated by many outcroppings and cliffs. More protected -fewer islands.
The rocks on the west coast haven't weathered down as much as the east coast. You also have to remember that a lot of sand on the east coast is imported.
There is sand but the Pacific Northwest Coast line is Mountain Range more then other areas
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