Why doesn't every text field check spelling?

I was editing a bunch of web and wiki stuff today. It's amazing what you find that looks dumb when you go back and edit a few days later. Sometimes, I can hardly believe what I write.

While fixing a typo in a web page today, I had that thought: Why doesn't every text field (like the Blogger web form into which I'm typing this post) check spelling while I type, displaying errors with the red wiggly underscore. MS Word did it first. It was a good idea. Now just about every application has implemented it. I get it in Thunderbird, for instance. So why don't operating systems (Windows) and graphics tool kits (GTK+ or Qt) implement this as a base feature, available to every application, whether they want it or not?

That way, my blog posts would be less typo-infested.