Why should the ideal temperature be right in the middle of the range?

It's no surprise that the maximum end of the range is right around the body temperature, as it's difficult for the body to keep itself cool once the environment is around or warmer than the body temperature. Sure, we can sweat, but that uses up a lot of water and people generally find that getting all sweaty to not be pleasant. Run out of water or raise the temperature too much and it gets dangerous pretty quickly.

On the other hand, if the environment is a lot cooler than the body temperature, then it is difficult for the body to keep warm. I'm sure for our distant ancestors who lived in what is now Africa, their minimum temperature was much higher, possibly putting the ideal temperature right around the middle of their range. Luckily for us, we have clothing and can put on more clothing to stay warm, which is how we can now make the minimum so low. But while we can use clothing to lower our minimum, we really don't have anything different to raise our maximum vs. our ancestors - we're both limited by how well we can cool ourselves by sweating. So for that reason it doesn't really surprise me that our ideal temperature is towards the upper end of what we consider the minimum and maximum temperatures.