Because the tech is changing so quickly that it is hard to recoup the investment before the camera is completely overtaken, maybe?
I look at the footage from my AF100 and 7D and compare it with the footage from more recent cameras and they really don't hold up very well. Hell, the 2.7K from my GoPro looks as good as the AF100 in bright light, and it cost less than 10% of what the AF100 cost me just three years ago. An iPhone shoots comparable footage in decent light. I know, ergonomics, ND, XLR, blah blah but eventually the numbers game becomes overwhelming. (Who is still buying SD equipment?)
We're in the "explosion" phase of video camera development that we went through a decade or so ago with stills, where camera generations came by every 18 months and demolished the generation before: 3.1 megapixels became 6 became 12 became 22 in a few years. We're out of the mad phase for stills cameras now, but just getting into the steepest part of the curve for video.
New video cameras are pushing to 4K and beyond. I'm a great believer in oversampling for image quality, so I definitely like shooting 4K to deliver in 1080p.
A few have in-camera RAW, or separate channel compressed RGB (eg RED, GoPro) or really good latitude (eg Arri, BlackMagic, REd Dragon) or stunning low light performance (Canon C300).
A few have XLRs and ND filters.
NOTHING has good ergonomics yet, apart from maybe Arris which lag behind on resolution. I know resolution isn't everything, but in a few years' time, those 1080p Arris are going to be as redundant as an SD camcorder, because everyone is going to be used to shooting 4K+ oversampling (even for HD delivery - same way no-one shoots with a 3.1 megapixel sensor stills camera any more, even for the web).
So... is it actually worth buying a video camera at the moment?
Obviously, if you don't have one and you need one, yes.
But I'm not sure it is worth paying big bucks for anything right now. I'd spend on glass and grip and buy a cheap-ass camera, same as I told people to do in the mad phase of stills camera tech explosion.
About the only thing that's got some future proofing is RED, and that's only because they were a bit ahead of the tech curve so it'll be a bit longer until my Scarlet is totally left behind and outshot by a sub-£1000 camera. Arri do too because they did a really sweet job on a HD camera which was point and shoot for film guys, and that'll keep their cameras in service for a long while. Who doesn't love the Alexa look, after all? But Arri will have 4K cameras soon and the original Alexas are going to look old.
If you really have a need for top-flight tech, I'd consider hiring unless you shoot several days a week and you're sure you can pay off tens of thousands of pounds investment in a couple of years, because that kit's going to be obsolete as hell by the end of 2015.