Because it's way easier than making one yourself! When we decided we wanted an infrared pen, we searched the internet for an hour and then gave up looking for one. We broke down and decided to make one. We drove to Radio Shack and bought a switch and our infrared light. Unfortunately, the battery was sold out. After about an hour of assembly, it didn't work. We searched for a company that would sell us an infrared pen once more, but couldn't find one yet again.
We did find out that you can take a regular LED pen and replace the bulb with an infrared bulb. We drove down to fry's, bought 3 LED pens, came back and forced the bulbs that were not designed to come out; upon inserting the infrared bulb, we closed the door to block the light. We saw no beam. The whole project cost $75 and we spent about three and a half hours of our time. As a result, we still didn't have an infrared pen!
After two weeks of searching we found a manufacturer that will allow us to sell infrared pens as an affiliate. Hopefully this will be what you're looking for and save you some time and money. We are also working with one other manufacturer for those that want a smaller infrared pen, to come in the future. In the meantime, good luck with your infrared pen!