I think the European health systems simply refuse to buy the relevant drugs at the prices being charged, and the producers refuse to reduce prices for them.

The US health system is criticized for being expensive, but some of that is from the US effectively bankrolling the development of these new treatments, which the rest of the world gets after some delay.

Exactly this. The hospitals will simply say that the treatments are too expensive and that they lack the funds, and you are on your own. In fact it's so bad that even basic chemotherapy is only covered to a certain point, and if you are not cured in that time, you have to pay additional runs of chemo from your own pocket, even though we have "national healthcare free for everyone who pays taxes".

As a doctor, why bother spending your time learning about exotic foreign cancer treatments that aren't available in your country, and your patients couldn't afford if they were?

It would be nice if doctors did anyway, but I can certainly understand why they wouldn't.