Why is the medical community trying to cure terminal illnesses?

It sounds like a stupid question to ask until you think about this important detail:

These naturally occurring terminal diseases are natural mechanisms to keep the population of the world under control. Without people dying of diseases like cancer, tuberculosis, and other such illnesses, the world would become so overpopulated that it would basically become hell on earth. Not enough food, not enough space, anarchy, etc. And that is something that would be a lot more difficult to solve than curing diseases.

So why are we bothering?
Why is the medical community trying to cure terminal illnesses?
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