An "haute-cuisine" cook can make business by keeping their recipes secret. That falls under trade secret.
The same principle holds for source code.
And for other industries, too. Cars, aircraft, hardware... The blueprints, or the sets of steps necessary to build those things, can be trade secrets.
Nobody suggests they can't sell the resulting products while keeping those processes secret. The only reason it doesn't make sense to copyright a dish (I mean the edible dish, not the recipe) is because those things aren't information, you can't take one of them and make a copy just like you can with software.
Software is peculiar in the sense that both the "recipe" and the resulting product are information - but they are not the same information, and there's no reason both sets of information should receive the same treatment.
Now, what doesn't make sense is to patent one thing and try to keep the same thing a trade secret.
For full disclosure, I am firmly against software patents as they are conceived in many jurisdictions. Patents should be detailed and should include the best method known by the author to carry out the patented process - for software patents, this means they should include the source code.