In the cave when Snow and Charming visited him he seemed to have to work hard to see the future — he squinched his eyes and screwed up his face in an effort to see that it would be on her 28th that she would save them. He didn’t know that by normal meas.
We also know that all magic comes with a price, so maybe he doesn’t just go around willy-nilly future-seeing all the time because it costs him something. It was worth it to do the prediction for Snow because Rumple was getting something out of it (her baby’s name).