Like software engineering. And like writing software, some people are better at it / enjoy it more than others. Software engineers and good salespeople are very well compensated because they're hard to replace.
> I have seen a lot of companies stating how important is sales, and how unique they are. It can't be true for all companies. It is like everyone thinking that they are above average, it is just not possible.
Most companies, particularly most start-ups, suck at sales. "Great engineers, shit sales" is a more common failure mode in Silicon Valley than "sold something couldn't be built." Also, "good at sales" isn't a one dimensional metric. There are different sales processes, some of which some people (and consequently companies) are better at than others.
> If your product is software, literally it is impossible that sales add more value than the engineering organization that builds the product
You're conflating how essential a step is to a process with how valuable it is. Mediating those factors is how difficult it is to replace the parts in the step. Consider, for example, an automobile assembly line. It would be incorrect to assume that just because the vehicle is useless without an engine, it is impossible for anyone to be more valuable than the person who mounts the engine.