(Re: Leaving KLAX): Because unlike in IF where people just fly direct to their destination across the ocean, in real world aviation there are designated entry and exit points for trips across the Atlantic and Pacific, including points in between. This is why when you look at a live image of Transatlantic flights on Flight Radar 24 or a similar app, you see them all in a couple single file arcs.
Only in IF do they just randomly fly across oceans at any point they choose.
To your original question, that space just south of the coast is Special Use airspace, with two airways south of that, which is what the flight plan in your post is using: