That's exactly why it shouldn't be done at any old whim. If the problems no longer exist, then there is nothing to incite that future time traveller to go back in time and make the problems cease to exist. It creates a paradox, and those have to managed very carefully or else you can start fraying the very threads of reality. It requires discretion and a highly-trained professional touch.

Besides which, if we decided to use our powers to remake history we, like any race, would surely do it to our benefit, not out of altruism for the downtrodden. It is a sign of Rassilon's wisdom that he set down laws forbidding us from following that dark path. [A path which Narvin saw in an alternative universe. It wasn't pretty.]