When you receive a birthday card, an envelope acts as a mere formality and a waste of a first impression.
If all future models were designed by D-BROS then special events would be very different. Okay maybe not very but still different.
The ingeniously designed envelopes show vehicles which all have an open tab. Once opened, the removed paper shows a car's exhaust fumes, a train's steam or an airplane's contrails.
If we were handed one of those, chances are we wouldn't care about the content. In other words, they'd be perfect for masking a bad card...