Earlier quoted context omitted.
Every day when I pass through the lobby of my apartment building, I think about the fact that our lettermail gets locked up tight in individually-keyed mailboxes, but — due to our building not being "large enough to qualify" for a parcel dropbox — large high-value parcels instead just get left lying on the lobby floor beside the mailboxes, with the matter of not stealing other people's parcels left entirely up to the…
A parcel is almost certainly replaceable stuff. A personal letter could be something like a passport or birth certificate or bearer bond.
Also, while many parcels are just purchases from companies, a parcel could very well be a priceless heirloom sent by a relative. Or, say, a live reptile.
The postal system bakes old assumptions into it, where ordering goods for delivery didn't used to be nearly so common, so a much larger fraction of parcels used to be of this personal variety. Which makes it even more curious that they're generally handled in such a blasé manner.
My only guess is that shipping parcels is expensive by itself, so almost nobody bothers to pay double to have their parcel handled securely as registered mail.