Cooper rejects the coast three times (SF/LA/SD).
Cooper suggests and/or accepts Reno/Yuma/Phoenix.
All coastal airports rejected. Only inland airports (all directly south) were acceptable/suggested.
Hmmm...I wonder why.
Maybe it's the size of the airport, although you won't really expect Cooper to say "sorry I'm going to jump out of the back here soon so I don't want the plane going near the coast so please just fly due south over Portland."
The evidence aligns with the, rather obvious conclusion, that Cooper didn't want the coast. Further, his 10k ft configuration means they can't go east over the mountains. There's only one way the plane can go...south. And what's south...farmland north of Portland. And the farmland has lights, unlike the forest above it. And what does Cooper do...he waits almost a half and hour while flying over the dark forest before he jumps right at the first metro lights of the farmland. It's all to much to be random. It's a plan and it worked.