There are many reasons but here is a big one:
According to the European Commission, by 2050 the percentage of Europeans older than 65 will nearly double. In the 1950s there were seven workers for every retiree in advanced economies. By 2050, the ratio in the European Union will drop to 1.3 to 1.
“The easy days are over for countries like Greece, Portugal and Spain, but for us, too,” said Laurent Cohen-Tanugi, a French lawyer who did a study of Europe in the global economy for the French government. “A lot of Europeans would not like the issue cast in these terms, but that is the storm we’re facing. We can no longer afford the old social model, and there is a real need for structural reform.”
In reality, they never could afford it, they were just deluding themselves.