Why doesn't the water vapor just stay over the ocean?

Because of the water cycles gravitational pull allowing it to go in gas particles

For clouds to form, air needs to rise, either on its own by becoming less dense than surrounding air or by being lifted by denser air impinging upon it. On a sunny day, land heats up quickly compared to the lakes. As air near the surface is heated, water vapor rises, cools and condenses into clouds. Over water, this same rapid heating of air does not occur. In fact, the cooler air over water typically moves inland in the form of a sea breeze to replace the air that has been rising over land. This influx of denser air, called a sea breeze front, enhances the lifting that occurs over land and can lead to the formation of more clouds.