I wasn't raised to send Thank You notes, but because I don't live in a vacuum I learned that this is the polite thing to do. People make a choice to care or not care about making an effort, that's what it comes down to.
For what it's worth, I am a little wound up on this front. My sister complains nonstop about how her spouse/our parents/(fill in the blank) puts no thoughts into gifts to her. She also complains bitterly when anyone gives her or her children a gift card. So despite living long distance from her, last three bdays went all over the place to build special packages for her bdays, to make up for what she felt she was lacking from others. Not ONCE has she ever even called to thank me for this. We'll talk, but I have to ask if she received it eventually, and I get just a "yes." And what does she send me...gift certificates. Plus, not once has she had my nephew/nieces send thank you for the long distance gifts let alone a phone call letting me know it's arrived. So now, I don't even bother. She gets a gift card, nieces get some cash in a card, and when my nephew turned 20 and old enough to know better that it was wrong to not even call with an acknowledgement, I stopped sending him anything all together. Since it obviously meant so little to him, why bother? This Christmas I made her and my bro's family some great gift baskets, personalized them so they weren't identical. Even I, who am so criticla of myself, thought they turned out great. My brother and his wife went on and on about them, my sister..in the SAME ROOM when they were opened, didn't even look at me or say a word. So now I'm done trying at Christmas. Next year it's a generic GC. Obviously she doesn't care, so why should I. Though sadly, I know she's now going to complain about me not "trying hard enough."
I have some sympathy for not sending thank you notes when the gift was received in person (except in the instance of weddings and showers). But to be fair, I don't think if that it's any kind of excuse to say, "well I was there in person, so saying thank you was enough." Following that logic, the giver could just say, "well since we were there in person, saying congratulations (or just showing up) was enough" instead of bringing a gift. If the "recipient" is fine with that, then it's all good, but it goes both ways.
Bottom line: if someone takes the time to go out of there way to find a gift for you, overall telling you they were thinking about you, it's a little petty to say in return you don't have the time or interest in taking a minute to pen a thank you. It's that extra effort that makes the difference.