Jud, in fact they were.
RAF Flying Boots (Pattern 1939)
Fighters like the Supermarine Spitfire in the background had no cockpit heating system. Some bombers (i.e., the Hampden) did have cockpit heating (although none for the gunners) but the systems were balky and often didn't operate.
Flying in an unpressurized plane at angels-10 or 15 or even 18 (i.e., 10,000 feet, etc) exposed pilots to sub-zero temperatures for hours at a time. Heavy fleece boots were required to forestall frostbite -- and didn't always accomplish that.
[You can tell it's a Spit by the shape of the cowling and the large aerial support behind the cockpit and the fact that the rear of the cockpit runs flush into the fuselage. The most beautiful plane of the war, thanks to its elliptical wings, and one of the most beautiful ever. Early in the war, no one was considered more dashing or heroic than a Spitfire pilot. You can be sure this isn't the only trim this fellow was getting (although a gentleman removes his pipe when receiving the other sort). On the downside, this fellow had about 30 percent chance of surviving the war.]