Some developers are just writing code to satisfy a spec and really could be replaced by any other developer; others are uniquely valuable and significantly increase their employers value, without necessarily being recognised for it.
Good software acts as a multiplier, so it's not equivalent to tightening bolts. It's more equivalent to finding optimisations that reduce the number of bolts which need to be tightened. A relatively small amount of software can drastically increase the value of a company/process
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.
Here's the thing, in corporate devs are as essential as people toghtening nuts in a factory.
The value one produces there is multiplyed by the internal know how, customer base, marketing reach and tight focus management, none of which it's brough by the developer.
That value is stripped from developer work just to be pocketed by management and sales is a myth, besides, if it werent there'd be plenty developers going solo and the median salary would be in the millions.
Edit: and lo and behold, instant downvote. No wonders the tech community is so blind and guillible when it comes to the value argument, from the extraction myth to the stock options gambling.