Except some children with SEN do come from families like that.
That's true of any group you could mention brasty.
The nearsighted, the flat footed, the bald. Any of them might come from troubled families.
Recruitment advertisements for opticians, orthopaedic specialists and trichologists don't carry a "difficult families" clause. (And they have more client contact.)
Why should one for an ECHP writer?
Especially as many of these families do abuse drugs and alcohol, and doing so in pregnancy puts you at higher risk of having a SEN
Are you using SEN interchangeably with "learning difficulty" there, then? Your ignorance (and if I may say so, your nastiness) is showing.
"SEN" encompasses a wide range of physical, sensory, developmental and learning needs. All of those children will need ECHPs if their need is strong enough. Some of them will be intellectually gifted. Many will go on to top universities. Others will go on to supported environments. Your generalisation is very narrow.