Treebeard I know this is a big complicated project but I feel like in China they would manage to build this in 1-2 years and and it takes us 16.
Rail Batlica does seem to take a really huge length of time from concept to completion.
I guess that at least a part of that is the fact that it passes through multiple countries, and even if they are all part of the EU, they still have to legislate, and seek budgetary and public approvals separately, as well as agree on things between them.
Having said that, single country projects in Europe, like HS2 in the UK are not super fast either.
There is also the fact that they have to acquire any private land in a way that is much more ethical than in China where the government probably just issues compulsory purchase orders and give property owners what they feel like for it (note: I don't actually know much about the land acquisition process for infrastructure projects and and am just making an assumption)
Plus all of the reasons that Arvīds pointed out. It is painfully slow, and there should definitely be ways to speed up these kind of projects that can make real quality of life and environmental changes.