1. Seed capital seems largely useless, my lifestyle is very modest but the realities of childcare and mortgage mean my living expenses are higher than for your typical 20-something. The sums of money available at an early stage wouldn't last long enough to build an MVP, demonstrate traction, then find an angel. I'd have to go back to freelancing, only then I'd have an investor on my back. Bootstrapping seems like the only real option.
2. Having worked for 14 years as a developer then manager, I have a pretty good professional network. I've been pleasantly surprised by how easy its been to get work that fits round my startup work. I'm booked till Apr 2013 and turning gigs down at the moment.
3. Having kids is a huge time commitment. I simply cannot work at the office for 80 hours or more like some startup people do. However, I share an incubator space with some people like that and without wanting to sound in any way arrogant, I've realised that (a) my time constraints force me to focus really hard, I put in 50 really focused hours a week whereas they sometimes kick back and chat for a while because their energy is low, and (b) I have a lot more experience than them so I'm able to move faster than them on things like how to prep for a meeting with a potential partner or how to architect a system for the future. I think of it like this: if I can't leverage my ten years extra experience on these people to match their output in less time, then I deserve to go bust.
3. Energy: I'm doing what I love, I have all the energy I need.
Basically, you have to be a different kind of entrepreneur, some wouldn't call my product a "startup" because it isn't (yet) a shoot for the moon type venture, but I'm still 'starting something up' and it's still an exciting challenge :)