I noticed, that in older films, 1940's, 50's & 60's, the actors most times wear a full suit or other heavy clothing and a hat even during scenes with high heat. Was there a reason for this?
The actors are/were wearing period appropriate clothing. A 3 piece suit was not uncommon when going to work or church. The suits were made from a variety of fabrics. Some fabric weights were meant for winter, others for summer. You will also notice that films from (and depicting) the 40's and 50's have adult women wearing dresses with hosiery (aka stockings). Period appropriate then, as well. It was not (generally) socially acceptable for adult women to wear slacks until the mid-60's.
There have been a lot of changes in clothing over the centuries. Less noticeable is the change in social norms and attitudes that accompany them. Yet they seem to go hand-in-hand.