A strip of plastic means you have to have a spring at that end of the battery, a hole in the device for the plastic strip, and enough structure around the batte

A strip of plastic means you have to have a spring at that end of the battery, a hole in the device for the plastic strip, and enough structure around the battery to hold the spring and battery separately. The hole also makes waterproofing more difficult. All of this would add weight and size, and those trackers are really small and really lightweight. Judging by the photos, the battery is just hard wired to the circuit, and the whole thing is encapsulated with a thin layer of epoxy for waterproofing. That's by far the cheapest, easiest, and most reliable way to build something as lightweight as possible.