Can someone explain why Cumulus clouds hardly ever form in summer over Lake Okeechobee during daylight hours?

Answer 1

Good observation. Here is the scientific explanation. The surrounding land surface heats more rapidly than the water of Lake Okeeechobee setting up a convection current which flows from the lake towards the land and then rises over the land. This same current produces subsidence or sinking air over the lake. This is essentially a land sea-breeze phenomena you are observing.

Answer 2

That is not only with Lake Okeechobee, but basically any larger mass of water. At night, however, land cools down fairly quickly, and the water keeps radiating heat, hence cloud formation above water at night should be more common.