His "competition" was a third down back and a blocking fullback. Maurice Morris and Strong also hardly ever carried the ball. There were seasons where Morris and Mack Strong had 50 carries between the both of them.
Sure, Alexander did great -- but the fact remains he did play behind one of the best run blocking lines in the NFL since the 90s Cowboy dynasty. Walter Jones and Steve Hutchinson were first ballot HOFers. Tobeck, the center was a Pro-bowler. I'd argue that there are a lot of running backs that would have looked comparable to Shaun Alexander in that situation. Even a few of his colleagues would have likely did something similar and of course one of them did in LT.
I think he was a good back, but it's also safe to say that his resume was inflated by playing behind giants.
Alternatively the linemen had the benefit of playing with incredibly talented running backs throughout their career, and for Seattle at least, in a west coast offense that favoured them in the passing game.
Walter Jones effectively blocked his entire career for elite running backs. 2008 was the only year he didn't block for one of Chris Warren, Ricky Watters, or Shaun Alexander.
Likewise Steve Hutchinson went from blocking for Shaun Alexander in Seattle to blocking for Adrian Peterson at the Vikings.
So were they the ones making the RBs great, or were the RBs the ones making them great? It's a bit of both, obviously.