From the UK, to the rest of Europe, the cost goes from about £10 to several thousand pounds. If you get cover at all.

And that's for countries which are mostly covered by the EU health insurance card (access to state medical care in Europe).

That sounds odd. I've bought emergency medical insurance for UK visitors to Canada several times, as recently as this summer, and there have never been any medical questions whatsoever.

Possibly because psychological disorders can be easily temporarily faked, thus permitting the purchaser to cancel their trip at will?

Maybe. They could include the need for hospitalisation for any cancelled trip refund. In the UK it's pretty hard to get into a hospital even as an informal patient.

There must be some reason though.

Less likely the vacation itself, but the "post vacation letdown" when one returns to everyday life.