Why, apart from the obvious reason that his name was at one point a big box-office draw and he's currently attempting to suck the last dregs from the addictive teat of fame, does more than one Terminator ever look like Arnold Schwarzenegger? Let's remember, these are meant to be top secret infiltration devices, made to go where humans congregate without drawing attention to themselves, and then unleash a hellstorm of violence when the humans have dropped their guard. Doesn't it kind of give the game away if the human guards manning the barricades say, "Oh, crap! It's the former governor of California again! Aim for the eyes, I always freaking hated 'The Running Man'!"
I know that they did show Terminators with different faces in the original movie (although they still had a certain muscular bodybuilder look to them. Really, if you're going to be cunning in your pursuit of the human race, make a Terminator who looks like Wil Wheaton.) But in the second movie, we go right back to Ah-nuld, even though there is no sane, rational or sensible reason for John Connor to send back an identical duplicate of the T-800 that Skynet sent to kill his mother, and several extremely good reasons not to. (They almost died trying to convince Sarah that it wasn't the same Terminator, they almost died fighting cops convinced that they mysterious cop-killer had made a return appearance...) Really, the only person with a sensible reason to look like Arnie in T2 was the T1000, who has a vested interest in impersonating his counterpart to sow confusion and dissent among his targets, and he never does it.
Really, the only place it makes actual sense is in T3, the one that gets mocked by people who weren't paying attention for all the plot holes it doesn't have. There, Skynet sent a Terminator specifically to use John Connor's affection for the Schwarzenegger model against him, and it was captured and reprogrammed and sent back to stop the TX. (Which, you'll note, was also designed to look skinny and vulnerable and not like what people picture when they see a Terminator, because that's what you'd do with an infiltration unit. Because T3 was a much better sequel than T2, but nobody remembers that because hearing Arnie say, "liquid metal" and "Hasta la vista, baby," in an Austrian accent was so cool that it wiped everyone's memory of how stupid the plot was.)
Unfortunately, it's too late to suggest that maybe the Terminator series isn't exactly begging for a return appearance from a washed-up 80s action movie actor, or at least that if they want one of those, Steven Seagal is much cheaper. Schwarzenegger has already been signed for Terminator 5: You Knew It Was Going To Happen, and that's final. But could we at least see something different done with him?