Why isn't extreme heat considered a disaster in the US?

In Hello Internet, CGP Grey's old podcast, he complained about Londoners complaining about heat waves. The heat waves seemed to come every year, right around the same time.

Saying something is a disaster is totally different from saying it falls under the purview of FEMA. Like cold snaps, heat waves are reliable, predictable weeks in advance, and a disaster only if our public infrastructure is already a failure. I realize the same could be said of something like Hurricane Katrina, but the greater scale and lesser frequency of such failures put them in a different class. I don't support stretching FEMA's already paper-thin resources even thinner.