Everything inside the car, including the air molecules, are all traveling at 75mph.
The air, and the insect, are not even aware that they are moving as they are not experiencing any friction with the road or the outside air.
On the other hand, the car tires and windshield are experiencing considerable friction with the unmoving roadway and outside air.
Ok, so the air inside itself is also traveling at 75 mph, which is why if someone smokes inside the car, and the window is cracked, it all goes whoosh (very scientific term, whoosh) out the window like airplane with a crack.
I think I understand. How simply defined. Thank you. That has bugged me for years.