(a) they don't
(b) because free anything is better than nothing
(c) the English language
(d) their relatives here
(e) they are deluded
A bit of all of the above, I suggest.
Honestly, if you are not convinced, just do a bit of googling about cancer survival rates and other such. We're usually bottom of the pile, or close to it. The ONE thing that's good about the NHS is that it's cheap, i.e. we spend less % GDP on it than some (not all) European countries do on theirs.
It's cheap(ish) and fairly crap.