But...no siblings are getting anything... It's not unfair!
I don't think most parents would be that nonchalant and dispassionate about their own children getting nice things in life - most of us derive great joy from wonderful things happening to our own children. Of course, we're happy when nice things happen to other children (although not necessarily so much to adult children), but it's in a completely different league when it's our own kids.
If parts are being given out for the school play and your child scoops the lead, I don't think many parents would say "Well, it makes no difference to me as none of us adults are being cast". Same if they're made Head Girl/Boy, win a contest, win a star prize, anything that isn't open to adults anyway.
Essentially, the majority of parents feel great happiness and a sense of real reward - achievement maybe - when our children feel happy - even when they're grown up. They are kind of an extension of us - not in a territorial or ownership sense, but that strong connection is always there; the fact that, although they are of course completely separate people, they came from us and their lives can reflect very much on ours.
A PP mentioned above about her FIL not wanting to leave his child-free DD anything, in case her DP ended up with it and it 'left the family' - but she is his family. The strong implication is that, if you don't have children, you're a 'dead end' and 'not really worth sharing anything with' in your own right.
Whether or not it's intended, it does very clearly pass on the message that you've failed the family by not having children - that your only intrinsic worth/legacy would have been your (non-existent) children, but as you don't have them, we've saved you the bother of having nobody 'worthwhile' to pass money on to when you die by ending the family wealth link before you, so you can just muddle along in your child-free existence until you yourself fizzle out, whilst we concentrate on our children who were productive.