Well, "real" driftwood is wood that has aged in water for quite some time.
For orchids - lets limit this to freshwater driftwoods as saltwater driftwoods are nearly impossible to use for orchids.

This means that the wood has been stripped down to the real hardwood and that it is devoid of any soft material that can easily decompose.
Its extended exposure to freshwater streams also cause some physical changes to the wood: they are significantly harder than when they originally started. Sort of a partial petrification.

The real beauty of a true driftwood is that "what you see is what you get". You should expect that the driftwood would still fade and decompose over time, but it would take a very long time before it loses its original shape and form.

Compared to green lumber for example that still has nearly all of its fresh material intact.
You may start out with a 2-foot piece of fat branch but after all the soft material has decomposed and faded, you might end up with a stick. This decomposition could be detrimental to the orchids if they end up attaching their roots to the wood, only to have that piece peel off or decompose - causing the root to lose its foothold.

Now, driftwood is not the only valid mounting material. It just happens to be the traditional favorite because of it natural look and rarely do two driftwoods look identical.

But there are no "laws" involving orchid mounting. So it is really up to the hobbyist what pleases their senses.
I have seen a brassavola in the past that was mounted on an old computer keyboard - it looks rather interesting.