Survivalist intentionally doesn't have high mag sizes with other weapons so it doesn't intrude too hard on perks that already have extended mags. That's part and parcel of the "jack of all trades" thing; if Survivalist got the same mag sizes that SWAT did, it would be yet another ding against SWAT, for starters.
And that's not even mentioning stuff like Survivalist's own weapons (100-round Killerwatt magazines just for existing, and it would make the Locust even spammier being able to output 12 shots before reloading). Or all the exceptions that they'd have to manually code in after the fact for Demo weapons so you can't get doubled mag size on top of Make Things Go Boom and spam even more. (Yes, I'm looking at you, Bombardier users.) Or Sharpshooter's weapons because haha 20-mag SPX and having to avoid the crashes from the game trying to process a 2-round Railgun/M99.
Survivalist already gets waaaaaay too much base kit and the last thing it needs is yet another completely unnecessary base kit buff for free. If you need it that badly, just mod it in or something. Or just pick alternate weapons. That's part of Survivalist, innit?