[wyattpeak]

Because most people have no idea what covers are, and unless you collect postage stamps the difference means nothing to you. If you'd asked me before I met a friend who collects, I would have assumed that using a stamp would render it worthless.

The article's talk page actually has a similar discussion over whether to call them stamps or covers in the title, so although they come down on your side its hardly a given.

[homero]

I still don't understand what a cover is. Is it the cancellation stamp?

[astura]

A cover is essentially an envelope (it got it's name because it covers the contents), usually stamped and those stamps cancelled.

Here's some examples of fancy covers:

https://store.usps.com/store/results/collector-s-zone-first-...

[meemo]

According to https://en.wikipedia.org/wiki/Event_cover they are "decorated, stamped and canceled commemorative envelopes"

Essentially they are envelopes with stamps and a marking over the stamps that prevents the envelope and stamps to be used.

https://en.wikipedia.org/wiki/Cancellation_(mail)