Because maybe the otakus are the ones who will splash out large sums of money on products? You're making a pretty big assumption, which is that MacF targets the random kid on the street. But the late time that the show is aired, the inclusion of some risque scenes like the vibrator phone in the locker-- all these indicate that the target audience is that very same otaku from the very begining. Hence you shouldn't be surprised when all these products come out-- I mean, you can find this even for more populist animes like Bleach and Naruto-- and it doesn't seem to have adversely affected their target audience.
Frankly, the anime industry is pretty mature, and I'd expect the executives to know where the money is. Maybe once (or if) Mac F attains the status of being a cult hit like SDFM then you will find T-shirts, etc-- just like they released during the launch celebration.
Ultimately, I understand that you are non-plussed by the release of such items-- but if you are, you can easily ignore them and just not buy them. But you should also understand that this is a business, and if there is money to be made in boobie mousepads, or a Macross shaped vibrator like what I think REbirth is talking about , you can be sure that someone will be there to make them.