There are approximately 296 million .COM domains registered. That’s a lot of domain names out on the Internet that are either already taken or just parked in some obsolete spot gathering dust and all kinds of age. The most common names like loser.com. Jamesbrown.com are already taken by net investors who resell the rights to the names. Can you imagine someone having http://www.elvis.com ? He’s just waiting on the highest bidder!
There are 900 possible combinations for two letter sequences. If you’re looking for “ET” then you just won’t find it! Even allowing for digits, again every single web address is taken. Of course, that’s ignoring the fact that .COM registrars now mandate a 3-character minimum length, so it wouldn’t be an option.