Because, in my limited memory, I didn't remember there being a sidewalk from LAX to, say, Century outside the airport. I don't have a problem with walking that far. I have a problem with walking that far, in traffic, when I don't remember there being a sidewalk. I had assumed that there wasn't one.

For example, walking into SLC airport is not trivial to do. It's designed to be driven to, and that only (or, now you can take the train). I mean, you can walk along the bike trail from the southeast corner of the airport, then work your way through the parking lots of the air cargo places, but I don't think there's a sidewalk from the bike trail to the terminal. And that's the best walking option at SLC.

[Edit: A bit of time in Google Maps shows a sidewalk the majority of the way in SLC, but not all of it.]