I think some people just think it looks like you couldn't be arsed to buy a gift/couldn't be bothered to find out what they might like - and look around for something. Just stuffing a tenner or twenty quid in a card is a bit CBA.
I expect some people don't mind it, and I bet many of kids don't, but personally, when I was a child/young teen, I preferred presents, not money stuffed in an envelope or gift card. At that age I didn't want or need money.
I don't want to go all Four Yorkshireman, but back in the 1970s when I was a child, me (and my brother and our cousins, who all have 6-7 years between us,) were happy with a lot less than children are now.
We were thrilled to get a board game, a selection box, a colouring book and some crayons, a pair of pyjamas, some slippers, an annual, some chocolate coins, a doll or teddy (depending on our age,) and the one big present, like a skateboard or a cassette player, or a walkman, or similar gadget that had just come out (that would be maybe £50 in today's money.) We never expected a lot, and wouldn't have been particularly chuffed with someone sticking a pound note in an envelope (like a tenner in today's money!)
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