Because it is hideous. UI elements should not mimic real-life materials. It's tacky and in bad taste, and people who like it are tacky themselves. Granted, I'm not saying you are tacky and have bad tastes, but take it as you wish.
At the moment I think Tiger looks a bit dodgy, some metal here, some grey plastic looking stuff there, some white plastic over there, and some stripey white plastic looking stuff at the back. Apple settle on 1 look! Also if we must have the grey and white plastic look (whatever they are called) can they at least look the same? The different sized corners make it look a bit weird.
I do like brushed metal. It has a certain elegance, and it's not as obnoxious as, say, those wood grain finish backgrounds. (I believe there are some skins for Mozilla along those lines. But what I would like more than brushed metal would just be unity across the OS. If you're going to make an app plastic, make them ALL plastic, fer Steve's sake*.
Yes, I do have Metallifizer, but it doesn't get to all apps. And Safari looks downright ugly when it's plastic-ized.
When iTunes went "plastic," I thought that I hated the new look. I really wanted my brushed metal back. So much so that I considered reverting to an old version of iTunes. But I'm lazy so I kept it, and I have to say that it has really grown on me! Now the brushed metal looks slightly silly in comparison.
Back in the system 7 - 8 - 9 days, it would drive me up the wall. Mac OS 8 had this new look (Platinum), but many apps like Quark and Photoshop still had those old-looking black & white menus.
Kaleidescope came to the rescue!!
Now we have pretty much alot of inconsistancy:
Brushed metal (Safari, Finder winows), Aqua blue (many apps like Office), plastic (Tiger mail) and "new" plastic (iTunes 6, iPhoto '06 etc.)
I assume Leopard will get the "new" plastic look, which I find nice
I know I'm way in the minority on this point, but I not only like brushed metal, I like having apps with different skins. Safari is brushed, Mail is white, etc. It helps me differentiate quickly between programs, especially since I usually have 6-10 open at once. Just one more way to make my life easier.
when i first switched to mac (circa june 2004), i found the brushed metal to be a welcome change from xp. now, what with all the inconsistencies, i much prefer to use Uno to unify my windows. that being said, when i use someone's machine that is not unified, i'm abhorred.
i agree with those who hope 10.5 sees a more unified look, but as long as there are hacks, i'm not going to be too concerned with it.
The BM looks OK on Safari as there isnt very much of it and it contrasts nicely with the web-page contents. A good figure-ground relationship between content and browser.
But i remember when the Quicktime player first had it on OS 8.5- god it looked terrible, especially with the crappy volume control. They've improved it somewhat since then, namly by making it smaller.
Yes, Apples UI inconsistancy is a little annoying. Stick one! but not BM. if you've used Mellel you'll know how bad it can look!
Next UI look- everything transparent make it really confusing and vomit inducing, but flash. MS will copy it in 3 years time for Windows Alps (or whatever ski resort they fancy). Alot of Windows users will have epeleptic fits and MS will get a bad rep and shut down!
I used to like it on the odd app here and there, but I now think that it is getting a little tired.
I also find that it sometimes overwhelms what is actually in the window, particularly if there are large expanses of it, which for me is bad design. I prefer the new white/grey plastic look as it is subtle and doesn't interfere with the content.