That's definitely true for some folks, but it's not universal. I split my time between contracting and my bootstrapped business. The business is profitable and operational costs are very low. The only financial risk is the opportunity cost of not being employed full time.
Through contracting I make about 30% less than I did as a salaried employee, but I am immensely happier than when I was employed full time. Even if I wasn't bootstrapping a business, that tradeoff feels worthwhile to me.
In your situation, no, your opportunity cost from working on a side project is probably bigger than you think. First of all, I wasn't talking about the contracting/employment choice, so you need to compare the time you lose for contracting.
And then you need to differentiate between cost and risk. You sound like you are comfortable with the cost you are taking on (good for you, nothing wrong with that!). But when talking about the risk, you need to compare it to the possible outcome that your business completely fails.
That's what I am talking about when I say that bootstrapping means you carry the complete downside risk for the business.
I don't know what kind of risk appetite you have, or what percentage of your total net worth got tied up in your side business. So it's completely possible that this is a rational and beneficial choice. But it's not as easy as "I think it's worth it."
And people also forget about the upside risk of outside funding. Yes, your piece of the pie shrinks, but the pie also gets larger.
And again, I don't want to make people feel bad about their decisions, I just wanted to point out that risk management, with respect to their own personal capital, is something that start-up entrepreneurs need to think more about.
Through contracting I make about 30% less than I did as a salaried employee, but I am immensely happier than when I was employed full time. Even if I wasn't bootstrapping a business, that tradeoff feels worthwhile to me.