Mind you - I'd love to see hard figures!
In California someone paid $120k Gross I think would receive $79548, before healthcare deductions (optional private healthcare in the UK is typically about $700 a year for a single person of working age, in the US I believe premiums (employer and employee) are more like $7000 a year)
Overall at the $120k range it seems a similar cost in the UK vs California. Office space in London is probably higher than in SF, but that doesn't apply for remote workers.
Context - We have a UK company with US parent, and choose to hire in the UK for this reason.
The market. The US market for high-salaried knowledge work always pays more, whether is Tech, Finance, Legal etc.
In general, if you're a high-earner/rich, America is a great place to be. If you're poor, the UK offers more support and protections, but this is a digression.
(Partially answering your "why", not commenting on whether the original statement is true.)
"the average health insurance cost for employers was $16,253 annually"
https://www.peoplekeep.com/blog/cost-of-employer-sponsored-h...
Now some employers in the UK add private health care as a benefit of course.
https://job-prices.co.uk/private-health-insurance-cost/
Average is £74 a month, or $1200 a year.
UK firms pay for private health insurance too, but that's supplemental insurance on top of NHS coverage.