The reason water in a lake does not mix to become the same temperature primarily stems from the properties of water in relation to temperature and density. Cooler water is denser than warmer water, which leads to stratification in lakes.
In this stratified layer structure, the denser, cooler water remains at the bottom of the lake, while the warmer, less dense water stays on the surface. This natural tendency for denser liquid to remain below lighter liquid prevents the water bodies from mixing completely. As a result, distinct layers of varying temperatures can be observed, especially during seasons when thermal stratification occurs, such as summer.
In contrast, other factors such as mineral content or pressure effects do not explain why mixing does not occur in the same way that density differences do. The influence of light penetration does affect temperature by warming the surface layer, but it does not directly account for the overall inability of different temperature layers to mix efficiently.