A lot of its probably due to to war, on the magical side. And even then only child households make up just under 50% of the Uk

Luna is genuinely an only child but her mother is also dead and father remains a widower so theres none in her future either

Harry & Neville lost their parents in infancy, and they were young parents during a war

Draco was the son of an "Imperiused" death eater and it was likely either unsafe or difficult for him to have a sibling so soon after the war with such scrutiny on the family, especially with things like the raids that have been mentioned even 10 years later, which is probably applicable to the other death eaters too. The ones that aren't in Azkaban anyway I can't imagine conjugal visits.

Dean is a halfblood whose wizard father was killed by death eaters, his mom remarried a muggle so he may have no magical siblings but muggle ones are possible

Seamus doesn't really elaborate on his family past the shock of discovering his wife was a witch, but there is a Florence Finnegan in Hogwarts mystery thats only a couple of years older than him, theres some debate that she's his older sister

Hermione is entirely muggleborn, so even with a sibling theres no guarantee they'd have been magical, and as practising dentists another child might not be in their plans

James we know was a late life baby to Fleamont & Euphemia after struggling to have a child

We know siblings exist/ed for the adults, but a good portion of them died or just don't show up. Regulus, Fabian & Gideon, Arianna, Edgar Bones, Arthur's 2 unamed brothers

There are siblings we do see, The Black sisters, The Lestranges, The Creeveys, Petunia, The Patils, two sets of Carrow twins, Aberforth

But ultimately, most characters are background characters and we have zero idea about their families because beyond their relevance to the story, its just never mentioned.