Games Workshop creates codecies with the aim of giving players worldwide a common set of rules that close the gap and minimize conflicts between two strangers playing Warhammer against one another. However, GW is, first and foremost, a large corporation looking to turn a profit. It is not profitable to include variant army lists, obscure characters and rules for absolute customization. Each rule added increases the burden on their production costs which in turn bites into their profit margin unless they raise the price. Writing and playtesting require time and money but they also incur an opportunity cost - the more time you spend on an awesome Guard codex the less resources are available for Dark Eldar and so forth.
In short codecies are a chore for GW. One they must perform to maintain a dynamic metagame and boost model sales. But whilst writing rules is all but loathsome to GW it is, for the average player, a pair of wings to help one overcome the obstacles placed in front of themed army lists and codex reinvention.
So you've recently read Dan Abnett's Traitor General and you can't for the life of you find rules for a 'Wirewolf' anywhere.
That's because you haven't written them yet.
You spent 16 hours converting a badass Great Unclean One to lead your Death Guard. Too bad he'll just have to 'count as' a Greater Deamon.
You spent a heap o' cash on alternate Forgeworld turrets for your Tau Hammerhead - shame they'll never let you field it in a tournament since it's not 'official.'
Grown men applying the word 'official' to toy soldiers is almost as ridiculous as grown men playing with toy soldiers in the first place.
The inspiration for countless armies is there in the fluff, ready to be manifested on the tabletop but we'll never get there with just codecies. Remember the Most Important rule. I have played with custom rules in pick-up games in 4 different countries, I have yet to find a player that refused to play.
So next time you're thinking man I wish I could field an 'X' army just do it. No one will stop you. It's likely more than one person you know has been wanting to do it for a long time. Rules are just words and numbers on a page - the rest is your imagination. And when everyone is playing with "custom rules" they won't be "custom" anymore, they'll just be rules.