Because the fly has evolved to be in concert with the laws of physics. I hope your head gets better soon

Splat fly with hand. Grog do and laugh. Ha! Ha! Ha-hah!

Karma?

@harrygoodqi
I would gamble to say that the reason the fly appears to stay near stationary in your automobile when you change the velocity of the automobile is simply due to either A) the low mass and high air resistance the fly's body causes its velocity to slow along with the speed of the car... you can imagine a floating soap bubble in the car might have the same experience... or B) the fly actually adjusts its flight to maintain in a stationary position relative to the automobile around it.

Along this same line of thinking is this video from one of my favorite you tubers:

[google.com]

Simplest answer, a fly is very small so the air inside the car slows down at the same speed as the car and the air resistance is enough to oppose the tiny momentum of the fly so it doesn't pass through the air easily enough to hit the windscreen.

the air moving around the car to the fly is like soup so it just gets blown away just like when you try and kill one with your hand. you get insects die on your screen but nothing like the amount if everyone coming at you hit instead of going around with the air.no action without a reaction at any level.

Not enough mass

Because it can fly. simple really unless you think about it.

Try the same thing with a 50 lb dog (ok at 20 mph) after it hits the dash - you will go out and buy a seat belt for dogs. And you know the old joke about a fly hitting the windshield - the last thing that goes though it's mind....? lol

Let's try it without the seatbelt and let the fly film you.

He can go anywhere he wants. Maybe he flew out the window.?

The fly's miniscule mass and air resistance?