Dinosaur

My friend asked me to go to ToysRUs today to get this dinosaur for her which her 4 year old son apparently must have for Christmas and is practically sold out everywhere so she couldn't find it and can't get to the ToyRUs. The woman in ToyRUs told me it was their most popular toy.
I suppose I just don't get it not being a parent but I'm genuinely curious as to how do so many 4 year olds all want a particular dinosaur so much that it sells out?! How do 4 year olds even know it exists and why do they want it so much? And why do the parents have to buy it? What happens if you don't get them the particular toy they want? And £40 for a plastic dinosaur??!!