Several reasons why there are no carbines or rifles for 357 SIG.
* Non-standard head size. May or may not be an issue depending on the platform design, but most current rifles use one of just a few head sizes for ease of manufacturing. Sure, because of its design and its separate bolt head, an AR could be made in 357 SIG, but would people buy it?
* Lots of people oooh and aaah about carbines for 9mm and 40 S&W, but few pull out their wallets.
* For the same amount of weight as an AR chambered for 357 SIG, you can have a 223. And the magazine will hold 30 rounds instead of 25 or so rounds.
* Since 357 SIG is kind of a specialty pistol cartridge, ammo costs are not in line with 9X19mm, so people will be hesitant to buy it because of the higher cost of ammo.
But there's hope! If you just gotta have it, you could easily have an AR15 set up for 357 SIG and have a companion to your sidearm. Wouldn't cost too terribly much more than a standard AR15.