That may be because the US is leading the way on drug development. After the drug has been developed, the drug company will be able discriminate on price to maximize revenue. I imagine the cost of getting approval to sell a drug in European countries that has already been developed and tested for the US is much smaller than the actual development.

It could be that without the US system, these drugs would not be available in European countries as well.

Mandating lower prices for drugs is unlikely to work as it hasn't worked in the past (Venezuela everything, US gasoline, etc).