I presume that's largely because flying planes requires large sums of cash up front the vast majority of the time. With medicine, it does seem to be more of a meritocracy, of course there are some who've benefitted from a private education etc. but overall it's presumably going to be on your grades, life experience and aptitude.
Naturally, vast numbers apply for these programmes, on the two occasions I've applied for cadet programmes (both had airline funding attached and odds of success of 300:1), I've been shortlisted and at that stage fewer than 10% of the competition is left. I'm far from being a superhuman though feel that funding is the hardest bit, if all programmes required no candidate funding it would be much easier for those without the requisite cash and much harder for those with.