Trying to fill a backlog to just keep a dev team busy is not what a backlog is for. It would be better for the business to have them rake leaves out front than add to a system something no user needs for the sole purposes of looking busy.
I think the assumption is that there are clearly problems to solve and useful work to do, but that useful work is not always well-defined enough to be ticketed and represented in the backlog as individual parallelizable work items.