As mentioned, LAX are popular flights in general. Just because there are seats when you are booking doesn't mean it'll be like that when it comes to day of departure. Remember that everyone else on a normal ticket will get to pick a seat, so you'll be stuck with the leftovers... which are usually middles.
If you buy basic economy to save a couple bucks, then don't complaint when you are assigned middle seats at the back of the plane in different rows.