How about, because we want to? It's not being done for reward or to avoid punishment, but to live amicably with one's neighbors. In other words, it's truly from the heart. Many so-called Christians, however, apparently only do it to achieve heaven or to avoid hell.
For thousands of years before Christ people were quite comfortably (how else would civilization have arisen?). And for a couple of thousand years after Christ a goodly portion of the planet has lived quite comfortably not knowing about Christ.
Now, you apparently seem incapable of doing the right thing without external supervision. However, do not assign your motives to others, especially when you lack evidence to support your claims.