Why can't find halo LEGOs?

Firstly, Lego is designed for kids, and not many young kids will play on a 16+ game. Also, Lego might not have the correct rights to create a Halo theme, e.g. the makers of Halo might be against the idea. Lastly, Halo is a video game and not a film, so Lego might not want to place Halo under one of their 'themes'.