I've noticed it's not always easy to find gamertags that are available. Many are already taken. It seems a bit silly to add numbers, hyphens or underscores to the gamertag to make it unique. The reason for this is that if you are then telling your friend your gamertag so they can friend you, they might spell it wrong and not be able to find you. In the case of numbers, there's a chance that YOU might not even remember the numbers correctly! Another issue is that underscores and hyphens aren't always the easiest to enter on a console (you may need to toggle the keyboard to punctuation for that). All of this I've seen myself in real life.
Yet, there really are still plenty of good 2-word gamertags available, it's just that no-one has time to really look anymore. I can relate; when you've just gotten a gaming system is not the time that you want to systematically hunt down gamertags that you might like. It seems like a waste of time, and your brain is already geared up to play the new game, not to think up usernames for yourself.
There are some solutions to this.
One is to use a gamertag generator. I tried out the Gamertag Generator Quiz at Quizony. It asks you a bunch of questions to try to personalize your choice of tag to you. The major downside is that you only get one tag after answering 29 questions(!) and dealing with some pop-up ads. If you're looking for something more fun to use, then try the free gamertag generator at SeaFlood - it'll give you several gamertags right away, and if you don't like them, you can generate a new batch. In both cases, there's no guarantee that those tags are available, but they're a good start.
After you get a shortlist of names, try a tool that checks the availability of gamertags on the system you're about to use. For example, in the case of the Xbox, you can use CheckGamertag. This is helpful if you'd like to do this before you get your new gaming system.
There are other tools out there to generate gamertags and check them - this is just a few here. But you don't have to settle for numbers, underscores or hyphens to get a unique gamertag. There are some good ones still out there!