I would think that "improving connectivity to local transit agencies" in Miami is pretty low on Amtrak's list of priorities, as it should be. The vast majority of long-distance train passengers aren't going to be connecting to the Miami transit system with luggage etc.
The real reason they need to get to the airport station is because the rental car center is there. And, in fact, right now, if you want to go to the airport station on the Silver Meteor or Floridian, you can do that by getting off at Hollywood and taking a Tri-Rail train to the airport station. Sure, it's a bit inconvenient, which is why, when I went to Miami, I got off at Maimi and paid $20 for a taxi to take me to the rental car center.
Maybe at some time in the future, Amtrak might run corridor trains in Florida. In that case, with more departures and a broader passenger profile, maybe connecting Amtrak to the transit system in Miami might become a higher priority.