There are a couple of things that you have to realize:
1) A site doesn't have to be all ASP.NET. I am rolling some new pages into my sites as ASP.NET pages, but the old pages are still ASP. There are several reasons for this. I developed some very sophisticated sitemap and page template technology in ASP that is used on virtually every page on the site. Similar technology will be in the box in the next version of ASP.NET, but I've had it working for years.
I can't abandon this technology without redoing the entire site, which means a lot of work, plus it changes the URLs of all the pages. All of the pages have been indexed in Google and many are linked from elsewhere. So I have reason to leave existing pages alone, but develop new stuff in ASP.NET.
If you look at the Microsoft web site, it is developed with a variety of technologies, increasingly ASP.NET, but not exclusively.
2) URLs that use the default page, e.g. [example.com...] don't show the file extension. So the page could be .aspx and you can't see it.
3) The vast majority of ASP.NET web sites are not public (and thus don't even count toward the 3 million that were indexed). If I had to guess, I'd say 9 out of 10 ASP.NET pages are not accessible from an anonymous Internet browser.
ASP.NET gets the most leverage when used for sophisticated intranet sites, and B2B (business to business) pages. B2C (business to customer) pages are frequently one-way interaction; B -> C. This isn't where you get the most leverage out of ASP.NET. If you are just posting an article on a web site, there isn't a strong reason to make it an .aspx page.
The B2C pages that do use ASP.NET are frequently pages that are database or XML driven. These pages are usually not the home page, so if that is what you are looking at, you will miss that the important part of the site is ASP.NET.