They're Built for Time Travel
But we're neglecting the whole time-travel aspect of the Terminator universe, and that's a central part of the plot: The main reason Terminators have to look and pass for human is so Skynet can send them, one at a time, back to the past, where they try to execute the leader of the resistance as a child, because he's too hard to kill in the present, when Skynet has access to millions of other Terminators, bombs, lasers and flying death machines.
John Connor only has the power of Scruff on his side.
Apparently Occams_Razor.exe failed to load on Skynet's last boot-up.
We'll roll with it, though: Terminators have to look human to blend into the pre-exploded world, and they have to be coated in flesh to travel backward through time, because in their universe time travel only works on living things. No clothes, weapons or artificial material goes back -- just flesh. So rather than just sit back and let the humans alter history, the Terminators found a loophole, coated themselves in skin and followed. That's the reason they can't bring back their crazy future weapons: They're not coated in flesh.
'Cause it's grooooss.
There's nothing exclusive about the human form that causes flesh to grow on it and only it. Animals, fish and birds all have living flesh, and they come in all shapes and sizes. Terminators have to be human-shaped to blend in, sure, but the only thing that lets them step through the machine unharmed and wake up in the past is that vat-grown flesh. So why not coat one of those sweet laser miniguns in a skin suit and bring it back with you? You've already grown flesh over metal in one shape; there's no reason you can't slap it on any shape you damn well please. Sure, it would look like a giant throbbing meat-dong until you split open its flesh and pulled the weapon out, and that's gross and disturbing -- but hey, this is war. It's not a pleasant business.
That thing doesn't fire gumdrops.
Or for that matter, why bother sending Terminators back to hunt John Connor down at all? The movies have established that Skynet doesn't know exactly where the Connors are at any one time. But by going through records, it can always pinpoint the city, county or at least general area. So we've already established that the steel of a Terminator's skeleton, the wiring and circuits of its brain, and its nuclear power source can all travel back in time covered in a thin layer of flesh. You know what else you can make out of those same approximate materials? A nuclear bomb. Why not just grab the biggest one you can find, wrap it in a skin-suit, beam it back to Bumfuck, Arizona, where you know Sarah is hiding out somewhere, and take out the whole damn state with your gargantuan, fleshy, nuclear death-testicle?
Something like this.
Jesus, Skynet. Do we have to do everything ourselves ... including genocide?
Hold on, we're not finished picking apart this franchise yet. Check out 5 Reasons The Terminator Franchise Makes No Goddamn Sense and The Mapped Out Terminator Timeline (Will Melt Your Brain).