Because WordPress.com doesn’t want to be a haven for blog scrapers and spammers and also because they, themselves are already running Adsense on our blogs. You can do a forum search for the issue: it’s been asked and answered several HUNDRED times already.
Your blog has Adsense. My blog has Adsense. They have an Adsense install on every blog at WordPress.com. You won’t see it if you’re signed in, or if WordPress.com recognizes you as a regular reader; usually people only see ads if they come to your blog via a search engine.
Like I said, there are hundreds of posts on the issue. A search should turn up a few screenshots of Adsense on the blogs.