I'm a bystander, but the gist I got from the official lists and trials is that each EU country needs it's own approval with the gov agency. It may be that "being EU" has equalized requirements to an extent, but each wants their own. (bureaucracy kills?)
One example I read about was a yet experimental drug made by a Swiss company, but the trials only run in and for the US for financial and market reasons.
While regulatory agencies may consider data generated from trials in other jurisdictions, they may ask questions about whether those data and conclusions and relevant and generalizable to their patient populations and health care systems.
There are also ethics to consider. Sure it may be cheaper to run trials in developing countries. But at what point does it become exploitative? Keep in mind that we are talking about human experiments, which is very serious business.