That's because they are something else.
They can't offer you a deal "power for your soul" because they are not interested in giving you power ("Real Ork takez power hm'self!") and are not interested in your soul ("Woz dad?"). Occasionally they can give a blessing to a powerful warboss or a weirdboy, but most of the time they are focused on trying to kill each other (one with cunning brutality, the other with brutal cunning).
They don't look for new worshippers - Orks believe in them naturally from birth and although there are un-orky traitors that worship Chaos gods... or even the Emperor ("He is da great git in Golden prizun!"), the wast majority believes in them unconditionally. Gork and Mork also don't have a huge plans to conquer the universe - they are happy to fight each other and occasionally help few greenskins in their Waaagh! (which can be best been described as "1/2 holy crusade, 1/2 pub crawl").
It is also speculated that they are not, like the Chaos gods, beings born in the Warp - instead they have been created by the weird psy-concious that all Orks have, which makes red cars go faster and their technology to work in the first place - simply Orks decided that there must be someone "dad is much bettur at cunning and bashing" and here they became (this is also the main reason you wouldn't call Emperor a Chaos god, you heretic!)
Also, apparently they could easily take the Chaos gods down... if they would stop fighting each others