Where's the drive-thru

When I arrived in London, my cousin and I were both starving so we were looking for a nearby place to eat. Relatively fast and easy. She wanted to find a drive-thru McDonalds so we could eat and run.... I was puzzled... aren't all the McDonalds and all the fast food places drive-thru? No, she said. In fact many of them aren't.
Hmmmmm.... that's strange. And we actually had to drive almost 45 minutes from Heathrow airport just to find a McDonalds.
In America there are drive-thru restaurants everywhere. There are McDonalds within ten minutes of each other literally. Some roads even have multiple McDonalds on one road... yes strange I know. And almost 99.99999999% of all fast food places (whether it is McDonalds, Taco Bell, KFC, Burger Kind etc... all have drive thrus). It is the America way. We eat and run. We drive thru and run lol.

Speaking of food though... the portion sizes in America are much bigger than in England and interestingly enough the price of food is cheaper in America than in England. I was with a good friend at a restaurant and we ordered dessert. And I was surprised that I was able to finish the dessert portion because it was actually smaller than the portion we are given in America which usually is enough for three people to share (certainly not something I could have by myself).

So why is this? Why do we have so much fast food in America? Why do we have an abundance of drive-thrus? And why is it so much more cheaper to eat out in America??