which is,
AFAIK, in all cases?
No, I know of 4 countries that don't require them either: Austria, Netherlands, Czech Republic and Poland (unless you're going to a towered field). It's in the AIP.
All the cross-country flight plan requirement serves is to find out which SAR is supposed to go looking for you. With today's technology (RADAR tracks, Mode S), you don't have to look at the flight plan's EET for border crossing.
Reminds of an old joke: Bavarian border patrol discover a dead body right at the border to Austria. It's a lovely day and they don't want to spend time with the paperwork, so they decide to move the corpse over the border to Austria. Some time later, the Austrian patrol walks by that place and the policeman exclaims: "Look, the dead man is back here again!".