What's called "seasoning" - and is a good thing - on cast iron is called "crud" on your other cookware. I'm not taking sides here, although I do agree with Ember, but couldn't you just as well ask why one wouldn't scour all the "crud" off your cast iron, since you do on the other utensils?
I always use hot water and a stiff brush on my cast iron - and have done for the last 40+ years - and once they're seasoned they stay that way. I did try washing a few of them in soap and water a long time ago and then had the "pleasure" of having to de-rust and re-season them....I haven't bothered with that again.
I guess it all comes down to what we think works for us: even if it's a placebo, it still makes us feel better!