You mean I have to say thank you?

I have teenagers and a toddler. My toddler says "thank you" all the time with no prompting. Not so with the teens. Last Friday I spent all day helping the 17 year old find a car. I spent two hours on the online auto trader, and then I drove her to the valley and we looked at some potential buys. We found something we both could live with, of course out of her price range, but, being good parents (or stupid, take your pick), we told her we'd help her out with the difference. My husband and I agreed to gift her $1000 towards the car and lend her another $1100 at no interest. She had herself $1600 in her savings. We didn't buy the car right there and then because I am not mechanically inclined, and I wanted my husband to look at it before we made a purchase. So, that evening we left the toddler with the 17 year old and my husband and I went back to look at the car. He testdrove it and decided that it was a good buy. So, we bought the car upfront and brought it home to our daughter. We walked in the door and said "Guess what? You have a car!" She didn't turn away from the TV when she asked "Which one did you end up buying?". She did not run out of the house screaming with joy that finally she had a car. We never even heard a thank you. She just asked how come we picked this one over the other one of the potential cars we had looked at earlier. All weekend she showed no excitement over having a "new" car....well it's a 2003 Hyundai....silver color with a sunroof. Did I tell you that my first car was a grey (not silver) 1985 Ford Escort station wagon with no airconditioning and no radio, and I was damned pleased to have some wheels (I was 25).
Yesterday, I was in a god mood and the kids were out of the house and the toddler was sleeping, so I decided to clean the teenaged girls' rooms and do their laundry for them. I am working up a sweat doing this when I get a text from the youngest teen "Be here at 4:40 at the latest to pick me up"....??? Hello, what happened to please and thank you? I have created monsters!!! But at least, once at home, the youngest teen noticed my work in her room and said "Thank you for cleaning my room". The 17 year old, however, did not. I physically took her up to her room and asked "Do you see something different about your room?" Her reply "You cleaned my room!" .....and.....??????? She gave me a puzzled look. How about a simple thank you, I replied. And I do take hugs as payments. In her defense I have to admit that she gave me a hug, But, what the hell, why is it like pulling teeth when it comes to get these kids to see that showing gratitude is a basic human response to kindness. How did I fail so miserably?