I wouldn't call it laziness but money-driven apathy. They probably tell the warehouse packers to use envelopes for all non-fragile items. As long as the cost of returns is less than the cost of hiring more packers to keep up, Amazon will do things this way.

And, from my experience as a warehouse order packer, the quotas are insane. I'm cautious with other people's stuff, so it wasn't long before they reassigned me. Commercial bulk buyers got better treatment.