Etiquette of a Door

Why is "thank you" becoming less common when you hold a door for someone?

I spent my lunch shopping in the city centre today...not for me, nor my wife, but for a friend of hers who is leaving her work this Friday. Sad times. Leaving 'I Love Candy' I was met by a young woman coming the other way so I held the door to let her in, but she just swanned in and didn't say anything. So frustrating!