It's only 91 and 92 that go all the way to Tampa -- 97 and 98 take the shorter route that only includes Orlando.

I think the simple answer, at this point, is that Orlando is such a big traffic generator that it's hard to imagine Amtrak bypassing it with the Florida trains.

(The complicated answer starts with the protracted strike against the Florida East Coast Railway that began in 1963, and includes a little of Amtrak eliminating Tampa as a maintenance base years ago, etc.)