Good question. I don't know. Just seems to be that way

Might be related to the fact that many carbs are a lot easier for your body to break down.

Even European cultures generally consume less fat and protein in the summer versus winter.

Hotter = less calories required to regulate body temperature so lighter foods. Plus eating heavy foods raises your body temperature because it spends more calories digesting the food itself.

As an example from the foods I grew up on (Ukrainian food), borshch in the summer is generally vegetarian and often consumed cold versus meat-laden in the winter...