A child from a rich family "being spoilt financially" is not actually "being spoilt" in my opinion.
For me, being spoilt is when a child does not appreciate what they have, have no gratitude, expect special treatment or to receive gifts for no reason, etc.
I don't think this is a trait just for only children though. Lots of only children are brought up properly, so that they appreciate what they have and don't turn into brats. Just as lots of children who have siblings are not.
To answer the question, I have no idea why some parents defend making their children spoilt, as if it's a good thing to do for them.
Yes, looking after your children emotionally and financially is important. It's important that they have everything that they need, it's important that they feel loved and wanted, and it's important for them to get "treats" sometimes... But it is also important to teach them discipline, and to not expect to be given things when they haven't earnt them. It's important to teach them to appreciate what they have, to appreciate what people do for them, and to teach them to have empathy for others and want to help.
Also, they need to be taught to be independent and self-sufficient to a certain extent. Needing constant attention and not being able to be okay in their own company sometimes is a bad trait, and isn't going to set them up well for life.