how come there aren't any disaster movies about a heatwave?

Check out The Day the Earth Caught Fire (1961) ... https://www.imdb.com/title/tt0054790/
Yup, another atomic bomb/doomsday scenario from my youth.
Is it any wonder why my generation can be so messed up?
"OK kids, time to dive under our little wooden school desks for (ahem) protection."
That being said, I must also admit that although I was only ten when I first saw the film, Janet Munro definitely gave me a hard-on. ;-)

Yes, it would be.
Which is why Hell https://moviechat.org/tt1643222/Hell is such a good movie.

Body Heat (1981) is not a disaster movie, but a crime movie. Hot weather is a background issue.