I worked (among other things) in tree planting as a teenager in Iceland. We weren't doing it very efficiently, but one reason we didn't have these huge bags is because generally your crew is planting this on land that has no existing trees, so you can just drive a pick-up truck alongside the crew.

Which looking at this Canadian video of the forestry service might explain the difference, i.e. if you're planting in an existing forest it makes more sense to carry everything with you: https://www.youtube.com/watch?v=UkjeC1rxRt0