Pretty easy to catch a shuttle bus to Disney and local hotels from the airport. Probably more transportation from there than downtown.
This is one of the many ways in which designing for cars/low density “curses” an area to perpetual underdevelopment: direct rail connections would be speculative rather than serving existing demand.
1) The actual city of Orlando is about 9miles (as the crow flies) from the airport
2) The Brightline eventually plans to go to Tampa on the west coast of Florida, but is coming from South Florida
3) The existing Amtrak/Sunrail lines run south-ish to north-ish on the west side of downtown Orlando. To use the existing routes and knowing where Brightline plans to go, it'd likely have to go through the airport anyway, then curve north to downtown Orlando, then turn around and head back down southwest-ish to Tampa.
4) At some point they hope to connect the existing Sunrail commuter rail to the airport, so you could take that to downtown
5) There's not really a whole lot to do in downtown Orlando itself, not a ton of shopping, restaurants or entertainment. Most travelers to Orlando are there to go to the theme parks that are outside of downtown.
So someone flew into Orlando airport, and then immediately took a train to Miami?
Why not just fly into Miami instead?
More generally, airports have many options for ground transportation and connecting to an existing transport hub makes a lot of sense.
Apparently there's a company that just runs shuttles to Disney.
Here's a driving map from the Orlando Airport to all the parks I mentioned above. You can see there's a central area not far from SeaWorld that could a good place to build this hub:
Disneyworld Paris has a normal station within it: https://en.wikipedia.org/wiki/Marne-la-Vall%C3%A9e%E2%80%93C...
It would have been a huge competitive win for Disney. People who took that route would not rent cars. Car rental, once you're on Disney property, is deliberately inconvenient, involving bus travel off the property.