No, it's not. There's a number of other cities that don't have lounge space or agreements at all that get considerably more service than IAD: SAN, SJC, LAS, DAL. Two of them are actual AS focus cities. LAS has something like 20+ flights a day combined between SEA/PDX/LAX/SFO. DAL gets service from ALL AS hubs and focus cities save ANC.

IAD isn't any different from BOS/BWI/EWR/PHL in being an East Coast city that AS serves, the only real difference being that there's no AA lounge, so AL card holders don't get lounge access (and I guess AS isn't inclined to do a deal with UA for AL card holders because, again, not a huge station). But "Why doesn't AS have pay-in lounge access in IAD for Golds/75Ks" is really small beer- especially as AS lounge access has never been a "hey, we'll let elites pay to get into lounges" model, it's been a "free access to OUR lounges if you paid cash or miles for first, 75Ks get some lounge passes, oh and we sell a lounge pass" model. AS doesn't care to change their entire lounge access model just because SRB has a nice lounge in IAD.