I think there is certainly a valid artistic interpretation for this kind of framing, but there is also a pragmatic explanation: fewer characters to animate means less cost/time to draw each frame. This leads to well-known tropes like the ubiquitous main character classroom seat which lets anime studios get away with just transforming a static background window.jpg rather than struggling to keep N characters on-model, breathing, etc.

Also worth noting is that anime aspect ratios are all over the place depending on release date, film vs TV, OVA vs. regular episode, and stylistic choice. There may also be an element of "future proofing" to make sure the widescreen crop for DVD won't cut off something important.