I'm not sure this is a universal experience.
The suburbs my family lived in were quite hostile towards pedestrians, as the lack of sidewalks had two primary outcomes:
1. People just rarely walked
2. As a result of 1, drivers became unaccustomed to pedestrians and adjusted to a driving style that assumed no pedestrians
I absolutely don't really fault the drivers too much, as it's reasonable -- the road is for cars and in most situations with roads it's not expected to have pedestrians except at crosswalks.
But it's not universal that suburbs are pedestrian friendly. Especially growing up in the midwest, even suburbs with sidewalks were not very pedestrian friendly. Despite laws requiring maintenance and upkeep of the sidewalks (including snow shoveling during winter), it's not feasible for able-bodied persons to walk across many suburbs because of how awful the upkeep of the public sidewalks are, neverminding people persons with disabilities or injuries.
The heat is inconsequential -- where I live now, since about May it's been 34º or higher real temperature (feel is close to 40º or higher), and yet everyone is accustomed to it and still out and walking. It's easily changeable, but the main difference is the city has many walkable elements, many small parks with shade, public fountains to cool off, and so on. Getting to a shop to get a cheap bottle of water (or beer, why not?) is a matter of walking 50 meters in any direction; the last US city I lived in, even though it was better than _many_ US cities, it was still almost 2 Km to get to the nearest store that had anything small to drink/refresh yourself (and you had to cross an 8 lane road that fed into I5, so it was pretty busy).
My takeaway as a US native is that US cities really did get ruined by the influence of car culture, and the cities are exceptionally hostile towards pedestrians. Even the quaint suburb-style cities ( < 500k population) which advertise their closeness and small town feel are ostensively pedestrian hostile just due to poor planning which prevents you from getting around by foot or by reliable, cheap, and safe public transportation.