If I want to travel from Paris to Marseille (500mi - like San Jose to San Diego) via TGV, it takes 3.5 hours.
When factoring flight prices and time to commute to the airport, it evens out. The only way you're trying a €40-50 flight ticket from Paris to Marseille is driving an additional 1.5 hours north of Paris to BVA, at which point the entire travel time is the same as TGV, with the added downside of dealing with Ryanair.
France is the perfect size to leverage rail transit nationally.
Edit:
Why the salty downvotes? Transit in France is different from Transit in the US.