If the human body runa at 37°C, why is it too hot when its 37°C outside?

I suppose, because it's 36-37°C thanks to a cooler environment. If the environment is also 37°C it struggles to keep itself cool, and the discomfort you experience is (also) to tell you to move someplace colder

The internal body runs at one temperature...

The external body runs at a different temperature.

And apparently testicles like it even colder, so they have attempted to move even further away.