When I booked a couple years ago, I think they all require taking a shuttle.
Just found this: All rental car companies currently operating at Sea-Tac Airport operate at a separate rental car facility, with dedicated shuttle buses operating on a 24-hour-a-day schedule. Two passenger pick-up areas are located outside
baggage claim at the north and south ends of the main terminal.
https://www.portseattle.org/Sea-Tac.../Ground-Transportation/Pages/Rental-Cars.aspx