The problem us that Spain uses a different track gauge than France, so connecting the countries is difficult.
> Since the beginning of the 1990s new high-speed passenger lines in Spain have been built to the international standard gauge of 1,435 mm (4 ft 8+1⁄2 in), to allow these lines to link to the European high-speed network.
(...)
> In general the interface between the two gauges in Spain is dealt with by means of gauge-changing installations, which can adjust the gauge of appropriately designed wheelsets on the move.
I didn't know that!
However the huge investment in HSR (longest network after China, but low utilization), has also come at the cost of the local and regional train network (which still use broad gauge, yes there are gauge changing trains, but the broad gauge network is languishing).