Because money is involved and people become very greedy sometimes when money is involved.

In many countries, advances in medicine and in nutrition have made for a much lower disease rate than these countries had 50-70 years ago. However, this same period is only 2 generations, so the cultures have changed only a little. The result is that many more children reach their adult years than can be accommodated on existing farms and villages. Needing farmland, they encroach and clear the wild rainforest as it is the only unclaimed land. From your point of view, this damages the environment, but from their point a view, they need to feed their families and it is the only life they know.
Another problem is that many societies in tropical areas survive by "slash and burn" farming. The soil in the rain forest is rich, and supports food crops for only a short while before becomming depleted. After that, it will not support a intensive farming. Techniques for replenishing the soil are largely unknown and the cost of artificial fertilizer is beyond the reach of these cultures-- so they burn more virgin forest to get the better soil, leaving spent soil behind.
People in a given culture often don't view their actions from a global context-- can you think of actions in your own culture that are wasteful or globally damaging? Hint: Look at fossil fuel usages by country or the ozone hole issue...
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People do destructive things because they value the benefits that they get more than the damages. In the United States, many people drive big polluting trucks even though they could drive small cars that don't pollute nearly as much. This is because they like big trucks and think that having what they like is more valuable than the harm they cause in pollution. This isn't because people are bad. It is just because the harm that they do is spread out among many people and not just on themselves. So rather than just one person being harmed a lot, many people are harmed a little. This is just the same way in the rainforest, except the people there are mostly poor and don't have many choices like US citizens. Most of the rainforest loss is from poorer people chopping down the trees for firewood or to grow food. Many people don't have a choice because there is no other way to get these things. They value the firewood and food much more than the rainforest.