In my American experience, flights these days are almost always completely full, and almost every passenger is bringing both an under-seat and overhead-bin carr

In my American experience, flights these days are almost always completely full, and almost every passenger is bringing both an under-seat and overhead-bin carry-on item. The last 10% or so of boarders are often left with no overhead storage near their seats. The last 5% or so of boarders are often left with no overhead storage available at all. "Put it in the first space you see" is often explicitly an instruction that the cabin crew gives to boarders as overhead space starts dwindling. Passengers aren't doing it out of preference.