By looking at it two ways:
1. Warner Bros. pretty much has everything concerning the North American rights to the Terminator franchise. They have Terminator 3, Terminator: Salvation, and Terminator: The Sarah Connor Chronicles. And if Time Warner ends up getting MGM, they'll own the Orion Pictures library which includes the first Terminator film. That gives them a lot of leeway to promote the series with pretty much everything except for the second film.
2. However on the other hand Terminator: The Sarah Connor Chronicles did abysmally in TV ratings even though it was given multiple chances. And Warner Bros had only the North American rights to Terminator 3 and Terminator: Salvation and those films really didn't do good domestically. And there's also the chance that Time Warner will not end up with MGM's film library therefore they're stuck with only the aspects of the Terminator franchise that hasn't done so well.
It's pretty clear that Warner Bros. looked at both of these viewpoints, however because they really didn't have anything to do with the success of the first film (if they even end up owning it), they don't have the second film in their grasps, and the lukewarm success to outright failure of the rest of the franchise, Warner Bros. went with the second viewpoint.
You also have to take into account of how many times the franchise has changed hands (James Cameron, Gale Anne Hurd, C2 Pictures, Halcyon Company) which doesn't look good. The fact that Halcyon originally wanted an absurd amount of money, and who ever wins would have to pay Lions Gate $450,000.
And a final factor is that Warner Bros. has plenty of properties to dig into without the need to buy another like DC Comics, WB Games (Mortal Kombat, F.E.A.R., Condemned, This Is Vegas, The Suffering, Spy Hunter, etc.), Looney Tunes, Hanna-Barbera, Harry Potter, Lord of the Rings, Nightmare on Elm Street, Friday the 13th, Sex and the City, Happy Feet, films by Chris Nolan, Zack Snyder, other various films, and if they acquire MGM, add James Bond, Pink Panther, Robocop, etc.
So yeah, I'd have to agree with you. Why would they?
Sony on the other hand handled the international distribution of Terminator 3 and Terminator: Salvation and while the last two films were duds in North America, they kicked ass financially overseas. And this time they would get all the money.
They also don't have a lot of franchises to dig into like Warner Bros. does. They have Spider-Man, but they're worried that they'll eventually lose it (hence the quick reboot when Raimi slowed things down). They have the Robert Langdon series. Ghostbusters. Karate Kid, Bad Boys, Men in Black. And Sony Pictures Animation (Cloudy with a Open Season, Chance of Meatballs, Surf's Up). That's it.
So the answer for Sony would be why wouldn't they bid on Terminator?