It is all opportunity costs.

You can vendor the drops, for "x" gold.
You can DE the drops, for "y" gold (which varies, but will average something long-term).

One of the two options is a gain over the option, by "z" amount.
Naturally, you'll go with the better option, all things being equal.

Whether you vendor the item, and purchase Infinite Dust.
Or DE it, and acquire the Infinite Dust that way.
The method of getting the dust doesn't matter, except that you use the more profitable approach.

Combine so much dust, with so much cloth.
Which averages 36 gold in costs.
And end up with something which averages 55g in value.
And you are up the difference.