I am lucky. I work for a very small consultancy (3 people plus occassional contractors) and am paid a fraction of our net income.

The arrangement was arrived at because the irregular income schedule makes an hourly wage or a salary a poor option for everyone involved. I’m grateful to work for a company where the owners value not only my time and worth but also value a similar work routine themselves.

Which is a shame because I bet most knowledge workers aren't putting in more than three or fours hours of solid work. The rest of the time they are just keeping a seat warm.

Spoken like middle management. If a knowledge worker is only putting in 4 hours they're either mismanaged or dead weight. Fire their manager and see if they are more effective, if not, then let them go. As a developer I routinely work 9 hour days without lunch and so do the others on my team and most people I've worked with as a developer. Myths like the 10% developer and lazy 4 hour knowledge workers are like the myth of the welfare queen. We really need to be more aware that when we complain about 5% of people that it becomes 100% to those outside of the field.

>As a developer I routinely work 9 hour days without lunch and so do the others on my team and most people I've worked with as a developer.

I've come across people like you and they don't produce as much value as they think.

40 hours/week is of course just an established norm for a lot of people and companies. But two 20 hour/week folks tend to cost more than one 40 hour/week person for all sorts of reasons.

source?

Well, for starters people probably want health insurance in the US which often starts at some percentage of full-time. Various other benefits. Then two people are probably just more overhead to manage than one. Though they may offer more flexibility.