Each carrier has their own reasons for doing what they do operationally. The big ones you’ve mentioned have a lot of owner operators doing city work so you’re likely noticing trucks that aren’t owned by the company and/or spec’d ideally for city work.
With regards to using 53 foot trailers in the city, sometimes the reason is efficiency. That 53 foot trailer may have brought a full load into the city and once it was emptied out they put it straight to work again doing LTL pick ups. (The big trailer was needed for the fore-haul not necessarily the back haul).
It could otherwise be done for capacity reasons. Smaller trucks / trailers have less capacity and that can complicate things; in terms productivity and meeting timelines. (If that truck has to run back to the cross dock to reload/empty out a few times a day. -Not every shipper has just one or two skids).
LTL certainly has its place. To say that “trucking should all be full loads” is ignorant. Trucking is a service. It follows the needs of the
customer. Sometimes they don’t need large quantities because they don’t have the space or maybe the product expires too soon.
