They were built before environmental regulation was implemented, the same as the US interstate system. None of those could be built today as cheaply or easily as pre-1970s. If at all.
None of them could be built today as cheaply, but all the projects built in those countries today are still dramatically cheaper than in the U.S., despite similar or even greater labor and regulatory costs. This has been well reported. The U.S. pays 2x or 3x more than comparable projects in comparable countries.
Ultimately the costs come down to politics. You can see the problem repeated dozens of times in this discussion--"I'd support X if and only if X was exactly what I wanted". That's not a recipe for compromise, and compromise is fundamental to efficient group decision making.
The costs balloon not because labor is too expensive, but because we take too long to build things and are constantly changing things as we go along. Regulations don't get in the way because they're overwhelmingly difficult or costly; regulations get in the way because the regulatory processes are hijacked and abused to stone wall projects.
We can't get things done in the U.S. because our politics are broken, especially at the national level, but even at the local level.
Looking at the dates when the high speed lines (AVE) were opened in Spain [1], save for one (1992), all the rest were opened after 2003, and most were opened in the last decade.
France keeps extending their network too... Same with other countries.
Then I have a misunderstanding of EU regulation on the matter. Is it entirely new right of way construction or upgrades?
In the US the last giant scale build-out was permitted before the EPA or Army Corps of Engineers had any regulatory teeth on the matter. Most projects since have been lane additions or incremental extensions. Loops have been built, but are politically easier since they only need to make one metro area happy. There's only been a handful of "new" interstates built since then (at a substantially higher cost). Much of the originally planned interstate system (that was to be a later phase) just never got built.
The cost of labor, the cost of environmental studies, the cost of materials, and the need for the project to show a profit.
All of these factors are much different in China.