Or because you have to spend an egregious amount of time doing biz dev, and accounting, and software, and PM work, oh and don't forget about Wives, Families.
Doing just one 20-week project a year (FT) could net you around $120k, which leaves the rest of the year to do whatever with the family.
Considering how much Rails does for you, that's crazy. Where is it like this?
I'm in near-rural Virginia and my last hourly consulting rate was over $200, and I'm not anything close to being an amazing Ruby developer.
What was the source of the client? If they went looking for a 'programmer', I'm curious why they weren't (like many others) lured by others who are cheaper per hour but worse in every other respect.
Whether I was billing by the hour or the week, I always estimated in a ballpark number of weeks a project would take, which translated to an estimated budget.
I think my current job will be my last salaried position, at least for awhile. I have a broad range of technical skills and can talk to potential clients, but have always been a bit nervous about taking the plunge (Where will my next client come from? Will I just find cheapskates who want some website for $500 total? Etc.)
It's not about the technology used. It's never been about the technology. You're selling yourself. You're selling your track record. You're selling you.
It's always about the solution and the peace of mind you bring to the client.
There are projects which have the technical complexity of the Rails 15 minute blog demo which are worth $100k to the right business.
Concrete example: for example, the central SEO tool for Bingo Card Creator is "a purpose-built CMS which is superior in only one hyper-specific way over WordPress." (WordPress doesn't have a "turn this blog post into a bingo card" button. BCC's code does.) That tool comprises about, hmm, 500 lines of code. It has a crank attached to it that can be turned by anyone who has done teaching. That tool plus $3,000 of teacher time equals more than $200k of software sales. BCC is very much not the only software company in the world that can get that scale of benefit out of that scale of software complexity.
Or, lets see, you know ActionMailer? You know Rake? You know cron jobs? You know how you could have a cron job fire a Rake task which would send an ActionMailer email? That plus a few hundred words of copy plus a three line if statement can increase the yearly recurring revenue of some businesses by 7+ figures. You can charge $150 an hour if you can write that if statement given a description of the desired behavior. You can charge $30,000 a week (representative number, not a ceiling) if you can people what that if statement should say.
Back when I was a consultant, Rails was one tool in my bag of tricks and most of my clients were B2B software companies. It is very much not the case that B2B software companies are the entirety of the solution space for "clients who a computer system could make a shedload of money for."