Your question is the same of "Why Google does not make food?" or "Why McDonalds does not make software?".
They could turn the WT in a big fast food company if you think just in money. But they don't want to abandon their purpose in first place.
WT purpose: Listen Obey and Be Blessed. Control, control, control....
Thanks for all your responses. I agree with the idea that they disfellowship so that they can intimidate or eliminate anyone who may disagree with them. The argument I'm getting is that their mission is to spread the "good news" and obey god's commandments. For the good of the greater number, they disfelloswhip in order to thwart anyone who may be trying to undermine their efforts in accomplishing that purpose much the same way McDonalds might fire someone who was publicaly promoting vegetarianism or speaking out publically or to their coworkers about how bad McDonalds food is.