I wonder if you can blame the climate and geography as some do in America. Bucharest is hot and sweaty in the summer (avg high 30 C, avg 68% rel H2O) and cold and snowy in the winter. The city lies on a flat plain, far from the mountains or the sea. Then you give people the opportunity to move to Amsterdam or Milan. Plus many other European countries have great infrastructure built up decades ago while Romania was undergoing economic crisis and revolution. It's hard to catch up.

The root of the issue might be that free migration in the age of modern transportation requires a shared tax base.

Yes that's my point, there is a shared budget so at least in some indirect way, the money a Romanian professional makes in another country may end up contributing to Romania's development and growth. I don't have number or what but this has to be taken into account.