The comparison with the EU is likely relevant, as the EU is essentially a single market for aviation. LH consolidated everything in Frankfurt, with minor, secondary hubs in Munich and Vienna. KL has a single hub in Amsterdam. AF has a major hub at CDG. They also serve ORY but mostly O/D traffic. Generally, unless one wants to fly the low-cost airlines (i.e., Ryanair and such), intra-EU travel between peripheral airports is directed to one of these hubs regardless of geography, with a few minor exceptions.

Take, for example, a connecting flight from Sofia, Bulgaria, to Kraków, Poland. The most convenient connections are via Frankfurt. The direct distance is 550 miles. Via Frankfurt, it's 1400 miles. But in the end, it does not matter because if all flights are on time, total travel time is 4.5 hours, and there is no way to beat this by road or otherwise. A hub location that would be geographically more logical, such as BUD, would reduce travel time by about one hour. But then you cannot offer the same frequency because BUD is a thin market compared to FRA.

In the end, my sense is that DL (and LH in the case of the EU) has determined that people prefer high frequency vs. saving 30-60 minutes of total flight time, hence the single hub model seems optimal. Of course, non-stop is best, but if you have to connect somewhere, a major hub with high frequency seems to be better than multiple thin-market hubs that reduce geographical distance but do not allow high frequency.

Moreover, as others have said, if the airline is more efficient in the core hub compared to a peripheral hub, total travel time might even be faster.