Children don’t get state handouts and pensioners qualify for state pension based on tax they’ve paid. As for severely disabled people who can’t work, they need state support. Not people with low level ADHD and Austin’s/Aspergers.
Why doesn’t it apply to migrants because the social contract is between British citizens and the British state? If a foreign national cannot sustain themselves, they should leave and if their circumstances change, reapply to enter the country. This is a v basic concept practiced globally. If I wanted to relocate to UAE, Australia and Thailand, do you think they’d be paying be benefits if I run out of money after 5 years?