In the United States, this is called eminent domain.
What is even the point of negotiating with private owners if the law allows the government to take whatever it pleases, as long as it provides just compensation?
I think that you aren't fully considering the phrase, "just compensation". If they just go to the courts and "take" the land, then they have to argue legally over the "just compensation" required. That can require just as much time and negotiation as getting agreement from the sellers directly.
If they get agreement from the sellers, then they don't have to argue just compensation in court. They've already agreed on it.
Even if this process is slower, it is less risky. If you invoke eminent domain early in the process, it is possible that the courts will award a high "just compensation" for the land. And you are guaranteed high legal costs. If you wait, you can use it just on a few properties, with lower legal costs. And the earlier agreements serve to set the price of the remaining properties. This makes it less risky.
In the US, the Kelo v. City of New London case allowed eminent domain in an expansive fashion. Which then caused a number of laws to be passed limiting it. A similar result is certainly possible in Europe. Use of power by the government can result in the government power being reduced.