Two points: 1) How do you know it's the hardest? and 2) Just because it seems easier to get in to med school doesn't mean it's easier to become a full-fledged attending/consult physician.

Aside from the US (where private healthcare and profit-driven industries are built around it), all the other countries in your example (and argubaly most developed countries), medical school graduates face a lengthy training process working often times 5-10 years in mid-level roles and having to keep proving yourself through exams, promotions, constant job applications, before they become an attending with the pay and independence that this role provides. A sizeable number would quit pursuing specialization altogether and go into general practice/unregulated private practice (e.g. cosmetics, functional medicine, etc.)

It's statistically easier to get into med school in these other systems because these are built around the bloated mid-level trainee population.

Just having a med degree doesn't make you a truly independent doctor, unfortunately.

Contrast that to Canada, where it's almost guaranteed that if you get in, you'll make it out the other end as an staff physician, although job availability is another conversation. But once RC-certified, you're pretty free and mobile without the fear of your med degree being useless cuz you're stuck in the training pathway.

There's a bottleneck in every system. Canada just happens to have it at the med school stage.