yeah also summer colds are a thing. Also, even in many areas with hot climates, the winter tends to be cooler and rainy (unless you’re right in the middle of the tropics where seasons don’t really work the same way), which means that people still end up spending more time indoors (especially since even 15 °C can feel cold to someone from a warm climate). It’s really just people packing together in less ventilated spaces which allows viruses to spread more (although with influenza I have also heard that the virus can’t remain viable outside as long in hot weather, and iirc this is even somewhat true of covid, but most transmission happens at fairly close range, anyway).