Internal knowledge is not held by the company it is held by individuals within the company. At best what you are arguing for is that there are enough people that for a given salary you can use to replace your existing people and can pick up what they are doing quickly enough that they are replaceable. That is not the same as your disrespectful assertion that your average dev is the same as a person who tightens bolts. Even the most simple development tasks require a significant amount of knowledge and at least a few years of experience that only a small number of the population can actually perform.

It's true that an individual can't do the same things a company can do. That's not some profound assertion. It's however impossible to go from that and assert that is because they are incapable of doing those things even if they had the funding to do so. That is to say that the marketing issue is more or less an issue of funding or money. That is what keeps that individual devs from capturing more value that is instead captured by companies or by managers, marketers, etc. As well as hiring additional people to form an actual company that performs the functions of a company. However a significant number of founders are developers, many of them including Paul Gahram and Mark Zuckerberg average developers that went to found multi-million to billion dollar companies because of funding. Now after the fact maybe you can argue that there was something special about them, but that's a post-facto assertion. They were average devs and if they had not gotten the funding would be relegated to your nut bolt tighteners not having been able to express their "true" potential.