Sometimes redundancy is good.

Also, since the outside envelope has no user-readable markings, it's assumed that you'll mix things up and put the wrong DVD in the wrong one. Or even lose an envelope and stick 2 DVDs (each in their own sleeve) in the same envelope, which the recommend if you've lost an envelope.

I suspect that a DVD in a sleeve in an envelope has higher odds of surviving the Advanced Facer-Cancellers, Mail Processing Barcode Sorters, Delivery Barcode Sorters and Carrier Sequence Barcode Sorters. (I assume NetFlix envelopes get to bypass the Multiline Optical Character Readers and Remote Barcoding System entirely)