Why do we need to conserve water? doesnt the water cycle keep it going forever?

Yes, but ... "forever" is a long time. There are fluctuations up and down. And those fluctuations can be more than plus or minus 50%, for more than thousands of years. In parts of the USA west, water deep underground took more than 10,000 years to aculate there, and people are using it up in less than 100 years.