Yeah, I was thinking that coastline wouldn't be ideal for shore diving. Perhaps all those shops inland on the interstate are catering to those diving lakes and quarries (are there many of those)? Either way, those inland shops don't seem to me to be very convenient to anyone. Especially if you lived on the coast, but even if you lived along the interstate and had to drive 20-30 miles to one of them, then go to a road to the coast and drive another bunch of miles round trip.
There are a few good lake dives, some of which are altitude dives, and many are cold enough to be considered drysuit only by sane people. On the eastern side of the Cascades, there are some lakes that are decently diveable, I don't think there is much to see in them though.
Most Oregon dive shops are naturally close to population centers, so for most people in the state, they can pick up gear at the shop 15 minutes away and then go.
I think a lot of people pick up their rental gear/get tanks filled on Friday, then drive 2.5-3 hours up to Puget sound for diving on Saturday. If people dive over a couple days, there are close by dive shops there to get your tank fills.
Oh, and there is this site:
Oregon Dive Sites - A catalog of SCUBA diving sites in Oregon