While a minority may be connecting to an international flight that operates from FCO but isn't offered from MXP, why would anyone else fly when there's such an excellent train frequency with Trenitalia from Milano Centrale to Roma Termini?
The rail journey takes a typical three hours 20 minutes. Having undertaken extensive recent rail travel in Italy (plus elsewhere in Europe and UK), Trenitalia offers a good city-to-city option, unlike airlines. Its trains are not as luxurious as an Austrian Railways' RailJet or German Railways' ICE, but they're still good and fast.
Travelling by air internally in Europe for short distances is environmentally damaging but more importantly, it's inconvenient, especially if one wants to see the many historic city sights for which many travel to Europe. The hassles of having to catch a metro or other train at each end to reach the city centre, variable airport security and a flight on an unattractive Airbus A319 compares to a seamless journey by rail where one isn't cramped. There's access to much larger toilets, more comfortable seating than typical airline slimline seats, a more generous luggage allowance that from observation no one seems to check (not that I abused it) and an on-board buffet car. Plus the joy of seeing scenery at eye level (though admittedly there can be tunnels, and some noise walls can partly obscure views).
No wonder the plane was half full. Every long distance train I was on in Europe was well patronised: many were full, even on routes with frequent trains.
How long do we give ITA before it's broke like its predecessor?