Around 50% of Nike's TOTAL cost is advertising. Now if you think about that for a second, in terms of how big the organisation is and how many products/units of those products they ship, it tells you something about how low cost their production is. You are talking around 4p to make your trainers, maybe less. They add value via a massive marketing machine and a mass market approach to selling, not by investing in niche markets.
Paintball is the flea on an elephant's ass when it comes to the sports goods and fashion wear market. Nike want to spread cost over huuuge numbers of units (hundreds of millions) and that just aint gonna happen. The only reason they would put money into paintball gear would be to achieve further brand penetration amongst particular social groups - and further enhance the sale of their mainstream items and brand position. It would be another advertising cost - pushing out their own version of everything is very easy with licensing, especially when making a profit is not the main objective of the activity. They give $140 million a year to Tiger Woods (alone) and that, for the time being, will generate more sales than a tick on paintball gear.