The reason people doubt global warming, is because it hurts businesses and businesses have a major self interest in tricking people to think its fake.
Some people are easily duped, thats your answer.
The scientific community doesnt have any real financial or self interest in global warming being real. The business community has a huge stake in proving it false. Its basically the same as when companies were trying to prove smoking isnt dangerous.
I don't get paid by anyone to point out over the last 500,000 years the earth has been warmer several times. There is nothing unusual about current temperatures in the history of the earth.
I would argue you're easily duped if you think climate did it's own thing for millions of years, but something magical happened in the last 100-200 years which shifts mother nature and climate to human activity.
So you think the climate warms and cools because it feels like doing so? Their's always something forcing it to change, a driving factor behind it. And there's pretty substantial evidence that humans are that factor, whether you choose to ignore that evidence or not.