I wonder if it has to do with expense. In order to include enough hops to get to higher IBUs, they have to get someone to drop $40-50 on a kit. That seems like a lot.
The first couple of kits I bought were in that range. Brewer's best kits. They were quite good and looking back on it with a couple years of experience, they both had all necessary ingredients (including sanitizers), but very good directions for someone who didn't know how to brew.
I haven't ever used any other kits, but I have looked at what a few other blokes have bought, and frankly a lot of the other kits are total crap, either directions, ingredients, or both. Which isn't to say there are no other decent kits out there.
It was a pretty easy step after the 2nd brew kit just to grab the ingredients to make a good extract recipe "on my own". Saved a few bucks and was exactly what I wanted (then I went all grain on my 4th batch, but let's not get ahead of ourselves).
A really high gravity beer or a DIPA, sure, that takes a LOT of extra ingredients and if the kit makers wants a profit, it would be really expensive. However, lets be honest here, swapping in an ounce of high bitterness hops and extending the boil to 60 minutes, doesn't add much cost. Say an ounce of Magnum or warrior at 60 minutes, an ounce of cascade at 15 minutes and an ounce of cascade at 1 minute and an OG around 1.055 would most certainly meet the deffinition of a real IPA and might cost all of $5-6 more in ingredients.