Hehe, no. It's because they're capable of building walls, and after much pre-alpha testing that was determined to be OP. So the reasoning is purely a mechanics thing, nothing to do with lore in this case. Same with most of the other robots you can't hack (though the number is small compared to those you can).
Yeah it's fun to unleash tunnelers that get swarmed by engineers
That you can do to your heart's content
I've gotten the B-99 Schematic, but no fabricators allow you to make it.
Apparently it only takes 300 matter
Oh interesting, so you can get the schematic! Well, I'm still going to remove that because yeah you're not supposed to be able to build them. Before release I did lower all the matter costs of fabrication because it was too hard before (also time costs were higher)--needed to make it a little more accessible. Will continue to tweak. Next I'd like to make parts fabricate in caches, in the same way you can produce multiple robots at once.
And to add to my earlier replies to you Arseface, thanks for bringing up the ESC to access help/game menu thing. Last night before bed I realized I really did need to add that one, too. I originally avoided it intentionally because there can be lots of ESC-pressing and you may accidentally open it, but I think everyone's better than that... and it will catch a few extra players who don't see the other prompts.
I've been using manual commands just to get Schematics for slightly better power sources and propulsion. This method seems fairly reliable if you spend about 5 minutes working on it. After that I go back to running around and blowing things up. I have to say repair stations have saved my ass a couple of times recently. I think I like them more than the terminals at this point
I'm glad you find Repair Stations useful! I personally never use them, and was wondering if anyone else would at all, but then I'm more of a non-stop run-and-gun kind of player myself...