I have to agree with you on the question and that is why I up-voted. It is never explained in the book, but I will get you some canon quotes to support my thoughts(later).
Less likely theory:
It is never said that these people don't have siblings.
Imagine if you are the sibling of Neville. Well you probably died when Neville's mother(if you were in the womb) got tortured.
If you are Draco's or a deatheater's child's sibling and you were a squib, then odds are you were killed.
If you were Luna's sister or someone's else's then you might of died when they were little.
Or you might be the grownup sibling or you might be a squib.
Either way you aren't that important to the plot and siblings don't mention their family every five seconds. Also, if you don't have that much screen time, then you aren't going have much time to mention your family.
More likely theory:
The wizarding population isn't that large, as people are dying and since a lot of people don't want to marry muggles they either die out or inbreed. So the decline in birth rate might be what is happening. I put more stock in this claim.
In the book, it says that wizards are less populated as they used to be and they are inbreeding. This means a decline in birthrate(partially the inbreeding). If the wizards are having trouble having kids then less people will be going to school. I mean it seems logical less kids that are born, means people are less likely to have multiple births. And therefore less people are seen, because less kids are alive.
"Most wizards these days are half-blood anyway. If we hadn't married
Muggles we'd've died out." —Ron Weasley
It might be because of declining wizard blood makes less wizards, so less students with siblings go to school.
Wizards that are inbreeding are going to have less kids.
"The pure-blood families are all interrelated. If you’re only going to
let your sons and daughters marry pure-bloods your choice is very
limited; there are hardly any of us left." —Sirius Black
But I will get some more canon quotes.If I can.
Offspring of biologically related persons are subject to the possible
impact of inbreeding, such as congenital birth defects.
So from here we can see that wizards that are inbreeding are going to have less kids.
Also, inbreeding can cause these problems.
Reduced fertility both in litter size and sperm viability
Increased genetic disorders
Fluctuating facial asymmetry
Lower birth rate
Higher infant mortality
Slower growth rate
Smaller adult size
Loss of immune system function
Read the bolded stuff that alone explains why a lot of people may not have a lot of kids.
A lot of the wizarding families are rich. A lot of rich people don't have a lot of rich kids.
Or they have reasonable wealth, not all of them has to be Malfoy rich. England is not a extremely poor country.
ONE of the paradoxes of human biology is that the rich world has fewer
children than the poor world.
So the reason for lower birth rate can be because Incest(inbreeding) and Wealth.