Northern brewer's instruction are some of the best...they show longer fermentattion/aging/carbonation times for each beer...up to several months.
But the answer is pretty simple, generally speaking kit manufacturers, especially kit an kilo manufacturers, are concerned with selling more and more kits NOT with the brewer making the best beer possible.
They know that if they say in the instructions to wait, they may loose some people to hobbies that have more instant gratification.
They also know that the time that a homebrewer will remain buying kits is relatively short...they know that after a few kits, the brewer will either give up, start brewing extract batches from recipes in books and places like this, formulate their own recipes, or go all grain...so they want to sell as many kits as possible to the new brewer before he moves on to bigger and better things.
SO they know that even their beer will taste better if you leave it longer...but they know that in the time you wait you will be reading and learning and be less likely to buy another kit...They can sell three or four kits to you if you follow their directions
in the same time frame that listening to us and waiting a month and bottle conditioning for another 3-4 weeks.
But Even Palmer says you should wait with kits...
Originally Posted by How To Brew
Leaving an ale beer in the primary fermentor for a total of 2-3 weeks (instead of just the one week most canned kits recommend), will provide time for the conditioning reactions and improve the beer. This extra time will also let more sediment settle out before bottling, resulting in a clearer beer and easier pouring. And, three weeks in the primary fermentor is usually not enough time for off-flavors to occur.
The two hosts of Australian Craftbrewer Radio (the FIRST beer podcast, with hosts with a combined 60 years of brewing no-how and 10 years on radio or podcast) have a couple of interesting pieces to improve kit beers...
There's this multi-part article (with sublinks in side the article.)
Improving Your Kit
April 22nd, 2007
The guys get “Down and Dirty” for the Kit and Kilo brewer with the simplist yet method of making a kit beer that tastes great. They also taste the underpitched beer experiment, and follow up on a brewers problem with under-atttenuation. Our beer superhero tries to save the love of his life - and Wonder-Mole, while we look into a beer belly experiment. More on how to say beer words, drink driving, beer laws and a quiz question will fill out the program, with a typical Aussie beer tale sung at the end.
Clicky to listen.
This is an ever evolving hobby...Places like this is where you find the most state of the art information/wisdom about brewing, because of the sheer number of us trying new things, hearing new things, and even breaking new ground and contributing to the body of info on the hobby...Look at some of that inventions that came out of here, and then ended up later in BYO articles by our members...