Plumbers have to spend unbillable time driving from place to place. $130 per hour is great but if each job takes 1-2 hours it could magically halve.

Ever walk into a half empty restaurant and get turned away because they're fully booked?

The ones I talked to were not interested in taking on projects further than a 10-15 mins drive. Please note that plumbers also add markups on the products they install (water heaters, etc) and charge higher rates for weekend/after hours emergencies. Nothing wrong with it, they just take advantage of the good market.

Not picking on plumbers, just pointing out that jobs in the software industry are not as plum as they seem to some posters here, compared to other jobs.

You're still underestimating the hours that are not billable. That plumber drives 15 minutes each way for a two hour job. That's a 20% loss of time. Plus it's not billable when the plumber has to go pick up materials from a hardware store. Nor is it billable when they deal with the overhead of running a business, maintaining accounts, dealing with bills, providing quotes, etc. Plus benefits are coming out of that $130. A self employed plumber working 40 billable hours/week is likely working more like 60 hours/week.

A lot of self-employed consultant/freelance programmers charge that or more, and are booked for months (on the same project..)

True, but most plumbers are self-employed, most programmers are not...