A Conundrum

Why is it that whenever I open the door to go outside or come in from outside, there are always two or more flies right there by the door ready to fly right into the house, but those same flies never go anywhere near the door again once they are inside so that there are always flies coming in, but never flies flying out? Same goes for the car, too. They never get near the window I so helpfully open for them to fly back out, but rather they buzz around the dashboard incessantly. And why is it so hard to get rid of them once they've come inside? No matter how many I kill, there is always another one buzzing around, even though I was sure the last one I squashed was truly the last one. I am sure flies have their purpose, but I do wish they could fulfill it outside my kitchen.