Either the chance is too low or they indeed cannot be obtained from caves, you would have to ask Kilgore.
Screenshot:
Thanks for the screenshot. Looks like another gun that is way too shitty for how rare it is. There's probably crafted Plasma Rifles that are way better than that, hopefully Kilgore will get around to upgrading all of the rare guns soon.
Seems to a common problem with rare weaponry that was not previously available in 2238 and was made available in Reloaded. The Gauss Rifle for example is worse in base damage than the Sniper Rifle (not counting ammo modifiers) and it doesn't even have the long range perk.
The Gauss Pistol (which was available in 2238, albeit rarely) is actually better than the Gauss Rifle because the damage is almost the same, they have the same perk (Accurate), the same range, the pistol only has 8 rounds less in the mag and it has a smaller Strength requirement (and the ability to be used with the One Hander trait for even less skill points needed).
So yeah, the list:
Gauss Rifle worse than Gauss Pistol, Sniper Rifle & XL70E3
Pulse Rifle isn't much better than any decent Plasma Rifle for how rare it is
Pulse Pistol has much too low range to ever be useful (should be at least the same as the Plasma Pistol ext. cap's 20, not 15)
Alien Blaster could also use a small range buff (it's only 10, could be 5 more at least)
G11 & G11E worse than Assault Rifle & XL70E3
Bozar is too rare to seriously build around with no BG alternative, same AP to single shot as it is to burst
Vindicator much worse than Avenger at this point, even before the buffs it wasn't very good in comparison since the ammo is so rare
Also take note that none of these can be crafted, which is also a direct reason as to why a lot of them are really quite bad/mediocre compared to crafted ones thanks to stats as well as availability.