Gee, why don't the humans go after the lizards instead? (spoilers)

That still doesn't really explain why they don't kill these creatures instead of spend all of the time, money, and innocent lives to feed them every night.

The original short story this movie is based on really explained the relationship between the creatures and the people involved in the conspiracy a lot better.

Here are a few points:

1) The creatures in the story are the ones that tell Kaufman about what's going on. So they are presented as having a higher intelligence and can possibly provide some kind of contribution, which is why they're kept around. In the movie, they're just these hungry creatures crawling around the subway tearing at food, and the driver is the one that explains their deal with the creatures. Apparently the creatures can somehow grant superhuman strength to their human followers, but they still appear as mindless eating machines.

2) In the story, the creatures seem to have some kind of mental powers they use to control humans. There was also a massive other creature referred to as "The Father of Fathers" that inspires some kind of "desire to worship" in Kaufman. He is practically hypnotized in some way by this creature that he starts walking toward it without even thinking about it. So that explains WHY this secret society even exists, the creatures are somehow mentally controlling these people to follow them. In the movie, the character inexplicably starts working as the new butcher, simply because "he's got nothing left." There's no sense of power coming from the creatures, it's just "the society needs a new butcher, that's you, no more tongue for you either. Have fun."

3) In the story, with the mental powers and hypnotism, you would assume that if someone headed down into the subway with some guns, the creatures would mentally convince the people not to shoot, or possibly even to commit suicide so they can feast on them. In the movie there's no explanation of that. Someone could just head down there, shoot them all, and then when the meat train shows up that night and they're all dead, the driver/butcher would just be like "Oh f---, now what am I going to do?"

4) In the story, it's kind of explained that these creatures use their influence over humans to control the world with people keeping their secret in positions of authority everywhere. In the movie, it's kind of just like "they were here before us, so we feed them." There's not the sense of importance of these creatures to the world like there is in the story, it's just like a group of people found some alligators in the subway and thought "Hey, alligators were around long before humans, let's kill people and feed them to the alligators every night and worship them." Instead of just calling animal control and getting them to remove the alligators from the subway tunnel.

So it's not a matter of not paying attention. The movie failed to explain the original relationship the secret society had with the creatures and why they decided to keep the secret and feed them every night.