It costs more to give disabled children the same quality of life as non-disabled children. Scope estimates it can cost more than 900 pounds a month. https://www.scope.org.uk/campaigns/extra-costs I have seen other sources saying 500 - I am sure you could debate the actual amount since it depends how you calculate it/what you consider a decent quality of life. But even things like transport can cost more. Unless you have talked to someone in that situation its hard to understand the extent to which every single thing can be impacted because we take so much for granted - even leaving the house.
Very often one parent can't work if the children are disabled. But even if one parent is working their career can still be impacted - often children with disablities are seriously ill and can have health emergencies. And maybe the working parent needs to take on things like picking up other children from school etc while the SAHP cares for the child(ren) with disabilities.
Your online calculator won't tell you that. Of course some parents will take the P. e.g.there have been cases where foster parents essentially treated foster care as a business and starved their children while pocketing the income**. That doesn't mean we shouldn't pay foster parents - just that we should crack down on abusive ones. Some parents are abusive/neglectful. But parents are more likely than anyone else to have their children's best interests at heart - statistically most parents are not profiting from their children and a benefits system that assumes they are would be an appalling one
**This is more common in America where there seems to be less regulation and foster care farms more of a thing.